clock-debug: Add support to query current parent.

Add a "parent" debugfs file for each clock that can be used to query the
current parent of a clock. The file contains the name of the parent clock
if it exists. Otherwise, contains "None".

Change-Id: I5ba1dcd344e9a2a1eec7f2bae18cec3f9b7b2338
Signed-off-by: Saravana Kannan <skannan@codeaurora.org>