clk: qcom: Add support for debugfs support

Add the clock operation list_registers which would print the register
content of a clock. This is supported for PLL/RCG/Branch/Gate clocks.
Also add support for list_rate for the RCG clock which would list all the
frequencies/rates supported by the SW.

Change-Id: I3b4cf83e776750d993d53331142223109bf0862e
Signed-off-by: Taniya Das <tdas@codeaurora.org>