net: phy: xilinx-gmii2rgmii: Removed hardcoded phy address 0 for bridge

Current code expects bridge phy address at 0 which is not correct
expectation because bridge phy address is configurable.
That's why update the code to read reg property to figure it out
where bridge is and use it in phy creation code.

Signed-off-by: Tejas Bhumkar <tejas.arvind.bhumkar@amd.com>
Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20230915045043.4167628-1-tejas.arvind.bhumkar@amd.com