spl: mmc: introduce spl_mmc_get_uboot_raw_sector

spl: mmc: introduce spl_mmc_get_uboot_raw_sector

Introduce a weak function spl_mmc_get_uboot_raw_sector, then platform
could have their own implementation.

Signed-off-by: Peng Fan <[email protected]>
Cc: Simon Goldschmidt <[email protected]>
Cc: Tien Fong Chee <[email protected]>
Cc: Marek Vasut <[email protected]>
Cc: Andreas Dannenberg <[email protected]>
Cc: Alex Kiernan <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Patrick Delaunay <[email protected]>
Cc: Miquel Raynal <[email protected]>
Cc: Michal Simek <[email protected]>