clk: Add support for clock list_rates ops

Add support to clock debugfs to be able to display
 - clk_rates_max which indicates the frequency to voltage mapping
   of a clock.
 - clk_list_rates, the list of clock frequencies supported by root
   clocks.
 - Also display the clk_rate_max associated with enabled clocks list.

Change-Id: I0a202af6f46c7cf164036d65487db5c40aab4063
Signed-off-by: Taniya Das <tdas@codeaurora.org>
Signed-off-by: Deepak Katragadda <dkatraga@codeaurora.org>