spl: Kconfig: support U-Boot load from raw NAND

spl: Kconfig: support U-Boot load from raw NAND

Commit 2a00d73d081a ("spl: mmc: Try to clean up raw-mode options") breaks
the boot of the BSH SMM S2 board. As stated in the commit itself, "Some
boards use this value even though MMC is not enabled in SPL, for example
imx8mn_bsh_smm_s2".

Support load of the U-Boot image from raw NAND sector. This is equivalent
to load from MMC raw sector.

Fixes: 2a00d73d081a ("spl: mmc: Try to clean up raw-mode options")
Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>