mmc: sdhci-of-dwcmshc: add suspend/resume support

Add dwcmshc specific system-level suspend and resume support.

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20200515141926.52e088fe@xhacker.debian


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>