net: gem: Remove zynq_gem_of_init()

This function was used for OF init before DM.
Remove this function as the part of move to DM.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jagan Teki <jteki@openedev.com>