doc: Add measured boot documentation

Briefly describe the feature and specify the requirements.

Signed-off-by: Eddie James <eajames@linux.ibm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

doc/usage/index.rst

@@ -14,6 +14,7 @@ Use U-Boot
    partitions
    cmdline
    semihosting
+   measured_boot
 
 Shell commands
 --------------

doc/usage/measured_boot.rst

@@ -0,0 +1,31 @@
+.. SPDX-License-Identifier: GPL-2.0+
+
+Measured Boot
+=====================
+
+U-Boot can perform a measured boot, the process of hashing various components
+of the boot process, extending the results in the TPM and logging the
+component's measurement in memory for the operating system to consume.
+
+By default, U-Boot will measure the operating system (linux) image, the
+initrd image, and the "bootargs" environment variable. By enabling
+CONFIG_MEASURE_DEVICETREE, U-Boot will also measure the devicetree image.
+
+The operating system typically would verify that the hashes found in the
+TPM PCRs match the contents of the event log. This can further be checked
+against the hash results of previous boots.
+
+Requirements
+---------------------
+
+* A hardware TPM 2.0 supported by the U-Boot drivers
+* CONFIG_TPM=y
+* CONFIG_MEASURED_BOOT=y
+* Device-tree configuration of the TPM device to specify the memory area
+  for event logging. The TPM device node must either contain a phandle to
+  a reserved memory region or "linux,sml-base" and "linux,sml-size"
+  indicating the address and size of the memory region. An example can be
+  found in arch/sandbox/dts/test.dts
+* The operating system must also be configured to use the memory regions
+  specified in the U-Boot device-tree in order to make use of the event
+  log.