mmc: add support for scheduling mmcqd on idle CPU

In order to boost mmc performance on various platforms,
add support for configuring whether set_wake_up_idle()
should be called on the mmc queue thread (mmcqd).
The decision will be set in each individual platform's
dts file.

CRs-Fixed: 787554
Change-Id: I3989d3f5b8228785e6d3bc49c7eb01ebf2fa2f38
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>