net: macb: In ZynqMP initialization make SGMII phy configuration optional

net: macb: In ZynqMP initialization make SGMII phy configuration optional

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>