spl: spi: override CONFIG_SYS_SPI_U_BOOT_OFFS via /config-property

spl: spi: override CONFIG_SYS_SPI_U_BOOT_OFFS via /config-property

For the RK3399-Q7, we need some flexibility (depending on the feature
set we include in the SPL stage and how large our SPI flash is) in
positioning the SPL payload (i.e. the FIT image containing U-Boot, ATF
and the M0 payload) in our SPI flash.

To avoid having to deal with this through different U-Boot images, we
introduce a the '/config/u-boot,spl-payload-offset' property node
allow it to override the default setting.

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