ARM: dts: Correct CM4 PHY MDIO address

The firmware patches the PHY MDIO address in the DTB to cope with
variations between board revisions, but the default for the CM4 PHY
is currently 1 when it should be 0.

Reported-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Phil Elwell <phil@raspberrypi.com>