mmc: sdhci-msm: Support Retention of SDCC Regs in CX Collapse

During the CX Power Collapse event, the SDCC registers contents
are not retained. Add the SW support to save and restore the SDCC's
registers content during CX collapse event so that the SD card
can resume to normal operation after CX collapse resumes.

Change-Id: I8b452eaea2591c455147c475eb2811e5ebd12ddc
Signed-off-by: Bao D. Nguyen <nguyenb@codeaurora.org>