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>