mmc: Schedule mmcqd on idle CPU

Schedule the mmc queue thread on an idle CPU
to boost mmc performance.

Signed-off-by: Zhao Wei Liew <zhaoweiliew@gmail.com>

Original commit: https://github.com/halaszk/Perseus-halaszk-universal5433/commit/b683c3b09e3abf9b8cdb3a5444499302a8be4180