msm: clock-debug: Add support to set a parent via debugfs

To enable debugging of clocks that implement the
clk_set_parent API, add support in debugfs to set
the parent like so:

cd <debugfs>/clk/<clock_name>
echo "parent_clock_name" > parent

Change-Id: Icf1781b52ab6a366951f23a3c7b6cc9445442201
Signed-off-by: Vikram Mulukutla <markivx@codeaurora.org>