clk: zynq: Use clk_readl/clk_writel helper function

Do not use readl/writel directly because the whole
clk subsystem is using clk_readl/clk_writel functions.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>