net: phy: xilinx_gmii2rgmii: Add check for external phy detection

net: phy: xilinx_gmii2rgmii: Add check for external phy detection

Add check if an external phy is detected or not before proceeding
further to config the external phy. This patch fixes the issue of
u-boot hang or reset, if u-boot is not able to communicate with
external connected over gmiitorgmii bridge.

Signed-off-by: Siva Durga Prasad Paladugu <siva.durga.paladugu@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>