mmc: msm_sdcc: Add sysfs attribute to update idle timeout value

Add a sysfs attribute to configure the runtime PM idle timer value

In some cases (eg. audio playback) the power consumption drops if
runtime idle PM timeout is set to 10sec. So this sysfs attribute
should be used to configure the idle timeout for SDCC per target.

The value must be greater than 5secs or else will have a major
performance hit.

Change-Id: Ia5a9a81ce46e9c66a325fd1ec1ccd2e0f89a981f
Signed-off-by: Pratibhasagar V <pratibha@codeaurora.org>