mmc: Ensure linux starts in eMMC user partition

uBoot sometimes leaves eMMC pointing to the private boot partition.
Ensure we always start looking at the user partition.

Signed-off-by: Philip Rakity <prakity@marvell.com>
Signed-off-by: Bruce Clemens <bpclemens@marvell.com>
Signed-off-by: Mark F. Brown <markb@marvell.com>
Signed-off-by: Chris Ball <cjb@laptop.org>