net: phy: broadcom: Make LEDs 3+4 shadow LEDs 1+2

CM4 uses BCM54210PE, which supports 2 additional LEDs, choosing LED3
for the amber LED because it shows activity by default (LED4 is not
connected). However, this makes it uncontrollable by the eth_led<n>
dtparams which target LEDs 1+2.

Solve the problem by making LEDs 3+4 mirror LEDs 1+2 (which is much
simpler than adding baseboard-specific overrides, but comes with a
risk of making one of the LEDs redundant).

See: https://github.com/raspberrypi/linux/issues/5289



Signed-off-by: Phil Elwell <phil@raspberrypi.com>