clk: zynqmp: Dont panic incase of mmio write/read failures

Dont panic incase of mmio write/read failures instead return
error and let the peripheral driver take care of clock get
and set failures.

Signed-off-by: Siva Durga Prasad Paladugu <sivadur@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>