clk: Add support for list_rates ops for clocks

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

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