ARM: keystone: rename clk_get_rate() to ks_clk_get_rate()

The KeyStone platform has its own clk_get_rate() but its prototype
is different from that of the common-clk (clk-uclass) framework.

Prefix the KeyStone specific implementation with ks_ in order to
avoid name-space conflict.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Lokesh Vutla <lokeshvutla@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>