mmc: sdhci-acpi: enable sdhci-acpi device to suspend/resume asynchronously

This patch enables sdhci-acpi devices to suspend/resume asynchronously.
This will improve system suspend/resume speed. After enabling the
sdhci-acpi devices and all their child devices to suspend/resume
asynchronously on ASUS T100TA, the system suspend-to-idle time is
reduced from 1645ms to 1089ms, and the system resume time is reduced
from 940ms to 908ms.

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