tty: xuartps: Fix build error when COMMON_CLK is not set

Clock notifiers are only available when CONFIG_COMMON_CLK is enabled.
Hence all notifier related code has to be protected by corresponsing
ifdefs.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>