clk: add/modify debugfs support for clocks

Update clock debugfs to support the below functionalities.
 - Allow enabling/disabling a clock.
 - Allow calling set_rate on a clock.
 - Display the list of enabled_clocks along with prepare_count,
   enable_count and rate.
 - Display the register contents of all the clocks which support
   this operation.

Change-Id: Ib67b3a3409c9e7d8adb710bb524f54f543abf712
Signed-off-by: Taniya Das <tdas@codeaurora.org>