arm: dts: zynq: Move crystal freq. to board level

The fact that all supported boards use the same 33MHz crystal is a
co-incidence. The Zynq PS support a range of crystal freqs so the
hardcoded setting should be removed from the dtsi. Re-implement it
on the board level.

This prepares support for Zynq boards with different crystal
frequencies (e.g. the Digilent ZYBO).

Acked-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>