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 <peng.fan@nxp.com>
Cc: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>
Cc: Tien Fong Chee <tien.fong.chee@intel.com>
Cc: Marek Vasut <marex@denx.de>
Cc: Andreas Dannenberg <dannenberg@ti.com>
Cc: Alex Kiernan <alex.kiernan@gmail.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Michal Simek <michal.simek@xilinx.com>