clk: mxs: remove the use of mach level IO accessor

It removes the use of mach level IO accessor __mxs_setl/clrl, and hence
removes mach header inclusion from clock driver.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Mike Turquette <mturquette@linaro.org>