mmc: sdhci-xenon: Add Xenon SDHCI specific system-level PM support

Add Xenon specific system-level suspend and resume support.
Especially during resume, re-configure Xenon specific registers
since registers setting will be lost in suspend if Xenon is power off.

Signed-off-by: Hu Ziji <huziji@marvell.com>
Signed-off-by: Zhoujie Wu <zjwu@marvell.com>
Tested-by: Jisheng Zhang <jszhang@marvell.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>