mmc: esdhc: Implement power management for ESDHC

For FSL ESDHC controllers, when entering sleep the controller will power off,
therefore the registers will lose their values, and the driver should save
the value of registers during suspend and write them back during resume.

Signed-off-by: Jerry Huang <Chang-Ming.Huang@freescale.com>
Signed-off-by: Jiang Yutang <b14898@freescale.com>
Acked-by: Anton Vorontsov <cbouatmailru@gmail.com>
Signed-off-by: Chris Ball <cjb@laptop.org>