serial_zynq: Migrate to Zynq clock framework

Remove hard coded frequencies and use Zynq's clock framework to obtain
the UART clock frequency.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
1 file changed