mmc: atmel-mci: add runtime pm support

Add runtime pm support to atmel mci controller.
Use runtime pm APIs to enable/disable atmel mci's clock.
Use runtime autosuspend APIs to enable auto suspend delay.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Acked-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
[Ulf: Fixed compile error]