ipq806x: Add "snps,dwmac" to all gmac compatible=

ipq806x: Add "snps,dwmac" to all gmac compatible=

This was introduced to gmac2 and gmac3 in 57ea767a53c1 without fanfare.
There's no indication of why it was added to those devices, but not to
gmac0 or gmac1. It was probably an unintentional omission. It should be
present on all four gmac devices.

This property is considered by
drivers/net/ethernet/stmicro/stmmac/stmmac_platform.c
stmmac_probe_config_dt.

Signed-off-by: Mark Mentovai <mark@moxienet.com>
Build-tested: ipq806x/ubnt,unifi-ac-hd
Run-tested: ipq806x/ubnt,unifi-ac-hd