At present the PHY address of the PHY connected to GEM is hard-coded
to either 23 (BOARD_PHY_ADDRESS) or 0. This might not be the case for
all boards. Add a new 'phy-addr' property so that board can specify
the PHY address for each GEM instance.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <1598924352-89526-12-git-send-email-bmeng.cn@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>