ramips: mt7621: fix usb init

In 4.14 reg-names is required to identify resources.
Otherwise init fails with:

[    5.900501] xhci-mtk 1e1c0000.xhci: invalid resource
[    5.910499] xhci-mtk: probe of 1e1c0000.xhci failed with error -22

Signed-off-by: Roman Yeryomin <roman@advem.lv>

target/linux/ramips/dts/mt7621.dtsi

@@ -342,6 +342,7 @@
 		compatible = "mediatek,mt8173-xhci";
 		reg = <0x1e1c0000 0x1000
 		       0x1e1d0700 0x0100>;
+		reg-names = "mac", "ippc";
 
 		clocks = <&sysclock>;
 		clock-names = "sys_ck";