net: zynq_gem: Add the passing of the phy-handle node

Add the ability to pass the phy-handle node offset
to the phy driver.  This allows the phy driver
to access the DT subnode's data and parse accordingly.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Tested-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>