refactor(mmc): export user/boot partition switch functions

refactor(mmc): export user/boot partition switch functions

At the moment, mmc_boot_part_read_blocks() takes care to switch
to the boot partition before transfer and back afterwards.
This can introduce large overhead when reading small chunks.
Give consumers of the API more control by exporting
mmc_part_switch_current_boot() and mmc_part_switch_user().

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Change-Id: Ib641f188071bb8e0196f4af495ec9ad4a292284f