ARM: keystone2: Use common definition for clk_get_rate

ARM: keystone2: Use common definition for clk_get_rate

Since all the clocks are defined common, and has the same logic to get
the frequencies, use a common definition for for clk_get_rate().

Reviewed-by: Tom Rini <[email protected]>
Signed-off-by: Lokesh Vutla <[email protected]>