Add proper PCIe init for MT7628/MT7688 SoCs

PCIe PHY needs different initialization on MT7628/MT7688 SoCs than it does
on MT7620.
However, LEDE (and OpenWRT) dts files have the PCIe node for MT7628/MT7688
as compatible with mt7620-pci.
We already can handle this properly in our driver, so we just need to add
compat strings to fbsd-mt7628an.dtsi and the PCIe driver.

Approved by:	adrian (mentor)
Sponsored by:	Smartcom - Bulgaria AD
Differential Revision:	https://reviews.freebsd.org/D6395

sys/boot/fdt/dts/mips/fbsd-mt7628an.dtsi

@@ -36,4 +36,5 @@
 		 * different values for the reg property
 		 */
 		reg = <0x10140000 0x10000>;
+	compatible = "mediatek,mt7628-pci";
 };

sys/mips/mediatek/mtk_pcie.c

@@ -203,6 +203,7 @@ mtk_pci_ranges(phandle_t node, struct mtk_pci_range *io_space,
 static struct ofw_compat_data compat_data[] = {
 	{ "ralink,rt3883-pci",		MTK_SOC_RT3883 },
 	{ "mediatek,mt7620-pci",	MTK_SOC_MT7620A },
+	{ "mediatek,mt7628-pci",	MTK_SOC_MT7628 },
 	{ "mediatek,mt7621-pci",	MTK_SOC_MT7621 },
 	{ NULL,				MTK_SOC_UNKNOWN }
 };