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.
 - Display the frequency to voltage mapping of a clock.
 - Display the list of clock frequencies supported by the root
   clocks.
 - Display the clk_rate_max associated with enabled clocks list.
 - Add a trace_clocks node to dump the current state of all clocks
   in the ftrace logs.

Change-Id: Ib67b3a3409c9e7d8adb710bb524f54f543abf712
Signed-off-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: David Dai <daidavid1@codeaurora.org>