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

This was introduced to gmac2 and gmac3 in 57ea767 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

target/linux/ipq806x/patches-5.4/083-ipq8064-dtsi-additions.patch

@@ -879,7 +879,7 @@
 +
 +		gmac0: ethernet@37000000 {
 +			device_type = "network";
-+			compatible = "qcom,ipq806x-gmac";
++			compatible = "qcom,ipq806x-gmac", "snps,dwmac";
 +			reg = <0x37000000 0x200000>;
 +			interrupts = <GIC_SPI 220 IRQ_TYPE_LEVEL_HIGH>;
 +			interrupt-names = "macirq";
@@ -902,7 +902,7 @@
 +
 +		gmac1: ethernet@37200000 {
 +			device_type = "network";
-+			compatible = "qcom,ipq806x-gmac";
++			compatible = "qcom,ipq806x-gmac", "snps,dwmac";
 +			reg = <0x37200000 0x200000>;
 +			interrupts = <GIC_SPI 223 IRQ_TYPE_LEVEL_HIGH>;
 +			interrupt-names = "macirq";