mmc: msm_sdcc: Power down slots after reboot

Some reboot methods (eg. adb reboot) may not power
down SDCC slots. This can result in cards being
in unexpected states after reboot. Explicitly reset
slot voltage regulator on reboot.

CRs-Fixed: 341840
Change-Id: I8cdb52bc58e34900378af468794760653f3babad
Signed-off-by: Oluwafemi Adeyemi <aadeyemi@codeaurora.org>