mmc: core: Add a debugfs entry to set max clock rate

Limiting the max frequency supported by host controller
to a certain value can be useful for testing power consumption
at various frequencies that are supported by the host.

Note: If the card supports less than desired value then the
frequency of operation would be limited to that frequency.

Usage:
mount -t debugfs none /sys/kernel/debug
echo <desired_frequency> > /sys/kernel/debug/mmcX/max_clock
cat /sys/kernel/debug/mmcX/max_clock

Change-Id: I9e9a7e368f56d8e16548780288211bd8775fd048
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>