part_efi: document device-tree binding for part_efi configuration

This adds documentation on the u-boot,efi-partition-entries-offset
property (which overrides CONFIG_EFI_PARTITION_ENTRIES_OFF, if
present).

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

doc/device-tree-bindings/config.txt

@@ -12,3 +12,11 @@ silent-console
 
 no-keyboard
 	Tells U-Boot not to expect an attached keyboard with a VGA console
+
+u-boot,efi-partition-entries-offset
+	If present, this provides an offset (in bytes, from the start of a
+	device) that should be skipped over before the partition entries.
+	This is used by the EFI/GPT partition implementation when a device
+	is formatted.
+
+	This setting will override any values configured via Kconfig.