spl: mmc: Introduce proper layering for spl_mmc_get_uboot_raw_sector()

Introduce two new weak functions, arch_spl_mmc_get_uboot_raw_sector() and
board_spl_mmc_get_uboot_raw_sector(), each of which can be overridden at
a matching level, that is arch/ and board/ , in addition to the existing
weak function spl_mmc_get_uboot_raw_sector().

This way, architecture code can define a default architecture specific
implementation of arch_spl_mmc_get_uboot_raw_sector(), while the board
code can override that using board_spl_mmc_get_uboot_raw_sector() which
takes precedence over the architecture code. In some sort of unlikely
special case where code has to take precedence over board code too, the
spl_mmc_get_uboot_raw_sector() is still left out to be a weak function,
but it should be unlikely that this is ever needed to be overridden.

Signed-off-by: Marek Vasut <marex@denx.de>