net: zynq: Add clk framework support to zynq ethernet driver

If available use the clock framework to set the tx clock rate of the
zynq ethernet controller.

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>