arm: mvebu: spl: Allow to build SATA kwbimage for 4K Native disks

Add a new config option CONFIG_MVEBU_SPL_SATA_BLKSZ for specifying block
size of SATA disk. This information is used during building of SATA
kwbimage and must be correctly set, otherwise BootROM does not load SPL.

For 4K Native disks CONFIG_MVEBU_SPL_SATA_BLKSZ must be set to 4096.

Signed-off-by: Pali Rohár <pali@kernel.org>
Tested-by: Martin Rowe <martin.p.rowe@gmail.com>