doc: Add new doc for soc ID driver model

Add a new documentation file for UCLASS_SOC and its usage to describe
the SoC Device ID framework that allows SoC identification and device
data matching.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/driver-model/index.rst

@@ -19,5 +19,6 @@ Driver Model
    pmic-framework
    remoteproc-framework
    serial-howto
+   soc-framework
    spi-howto
    usb-info

doc/driver-model/soc-framework.rst

@@ -0,0 +1,68 @@
+.. SPDX-License-Identifier: GPL-2.0+
+.. (C) Copyright 2020
+.. Texas Instruments Incorporated - http://www.ti.com/
+
+SOC ID Framework
+================
+
+Introduction
+------------
+
+The driver-model SOC ID framework is able to provide identification
+information about a specific SoC in use at runtime, and also provide matching
+from a set of identification information from an array. This can be useful for
+enabling small quirks in drivers that exist between SoC variants that are
+impractical to implement using device tree flags. It is based on UCLASS_SOC.
+
+UCLASS_SOC:
+  - drivers/soc/soc-uclass.c
+  - include/soc.h
+
+Configuration:
+  - CONFIG_SOC_DEVICE is selected by drivers as needed.
+
+Implementing a UCLASS_SOC provider
+----------------------------------
+
+The purpose of this framework is to allow UCLASS_SOC provider drivers to supply
+identification information about the SoC in use at runtime. The framework
+allows drivers to define soc_ops that return identification strings.  All
+soc_ops need not be defined and can be left as NULL, in which case the
+framework will return -ENOSYS and not consider the value when doing an
+soc_device_match.
+
+It is left to the driver implementor to decide how the information returned is
+determined, but in general the same SOC should always return the same set of
+identifying information. Information returned must be in the form of a NULL
+terminated string.
+
+See include/soc.h for documentation of the available soc_ops and the intended
+meaning of the values that can be returned. See drivers/soc/soc_sandbox.c for
+an example UCLASS_SOC provider driver.
+
+Using a UCLASS_SOC driver
+-------------------------
+
+The framework provides the ability to retrieve and use the identification
+strings directly. It also has the ability to return a match from a list of
+different sets of SoC data using soc_device_match.
+
+An array of 'struct soc_attr' can be defined, each containing ID information
+for a specific SoC, and when passed to soc_device_match, the identifier values
+for each entry in the list will be compared against the values provided by the
+UCLASS_SOC driver that is in use. The first entry in the list that matches all
+non-null values will be returned by soc_device_match.
+
+An example of various uses of the framework can be found at test/dm/soc.c.
+
+Describing the device using device tree
+---------------------------------------
+
+.. code-block:: none
+
+   chipid: chipid {
+        compatible = "sandbox,soc";
+   };
+
+All that is required in a DT node is a compatible for a corresponding
+UCLASS_SOC driver.