doc: document /config/u-boot, spl-payload-offset property

This adds documentation on the u-boot,spl-payload-offset property
(which overrides CONFIG_SYS_SPI_U_BOOT_OFFS during the SPI loading in
the SPL stage, if present).

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