dm: Add callback to modify the device tree

Certain boards come in different variations by way of utilizing daughter
boards, for example. These boards might contain additional chips, which
are added to the main board's busses, e.g. I2C.

The device tree support for such boards would either, quite naturally,
employ the overlay mechanism to add such chips to the tree, or would use
one large default device tree, and delete the devices that are actually
not present.

Regardless of approach, even on the U-Boot level, a modification of the
device tree is a prerequisite to have such modular families of boards
supported properly.

Therefore, we add an option to make the U-Boot device tree (the actual
copy later used by the driver model) writeable, and add a callback
method that allows boards to modify the device tree at an early stage,
at which, hopefully, also the application of device tree overlays will
be possible.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Stefan Roese <sr@denx.de>

dts/Kconfig

@@ -14,6 +14,16 @@ config OF_CONTROL
 	  This feature provides for run-time configuration of U-Boot
 	  via a flattened device tree.
 
+config OF_BOARD_FIXUP
+	bool "Board-specific manipulation of Device Tree"
+	help
+	  In certain circumstances it is necessary to be able to modify
+	  U-Boot's device tree (e.g. to delete device from it). This option
+	  make the Device Tree writeable and provides a board-specific
+	  "board_fix_fdt" callback (called during pre-relocation time), which
+	  enables the board initialization to modifiy the Device Tree. The
+	  modified copy is subsequently used by U-Boot after relocation.
+
 config SPL_OF_CONTROL
 	bool "Enable run-time configuration via Device Tree in SPL"
 	depends on SPL && OF_CONTROL