In the macb binding documentation "phys" is an optional property. Make
implementation in line with it. This change allows the traditional flow
in which first stage bootloader does PS-GT configuration to work along
with newer use cases in which PS-GT configuration is managed by the
phy-zynqmp driver.

It fixes below macb probe failure when macb DT node doesn't have SGMII
phys handle.
"macb ff0b0000.ethernet: error -ENODEV: failed to get PS-GTR PHY"
Signed-off-by: Radhey Shyam Pandey <radhey.shyam.pandey@xilinx.com>
Reviewed-by: Michal Simek <michal.simek@xilinx.com>
Signed-off-by: David S. Miller <davem@davemloft.net>