spl: mmc: Factor out eMMC boot partition selection code

Factor out eMMC boot partition selection code into
default_spl_mmc_emmc_boot_partition() function and implement
weak spl_mmc_emmc_boot_partition(), so that architecture or
board code can override the eMMC boot partition selection.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Faiz Abbas <faiz_abbas@ti.com>
Cc: Harald Seiler <hws@denx.de>
Cc: Lokesh Vutla <lokeshvutla@ti.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Peng Fan <peng.fan@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>
Cc: Ye Li <ye.li@nxp.com>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>