mmc: core: enable mmc host device to suspend/resume asynchronously

This patch enables mmc hosts to suspend/resume asynchronously.
This will improve system suspend/resume speed. After applying
this patch and enabling all mmc hosts' child devices to
suspend/resume asynchronously on ASUS T100TA, the system
suspend-to-idle time is reduced from 1645ms to 1107ms, and the
system resume time is reduced from 940ms to 914ms.

Signed-off-by: Zhonghui Fu <zhonghui.fu@linux.intel.com>
Acked-by: Venu Byravarasu <vbyravarasu@nvidia.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>