mmc: sdhci-xenon: add runtime pm support and reimplement standby

Enable runtime pm support for xenon controller, which uses 50ms
auto runtime suspend by default.
Reimplement system standby based on runtime pm API.
Introduce restore_needed to restore the Xenon specific registers
when resume.

Signed-off-by: Zhoujie Wu <zjwu@marvell.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>