serial: xuartps: Cleanup the clock enable

The core handles the clocking now. Remove the clock disable in
suspend. In resume we enable the clocks and disable after register
write.

Signed-off-by: Shubhrajyoti Datta <shubhrajyoti.datta@xilinx.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>