ramips: dts: mt7621: add the missing XHCI power regulators

ramips: dts: mt7621: add the missing XHCI power regulators

The USB power regulators are essential for the Mediatek XHCI
controller. If any of them is missing, the kernel will throw
a warning. Add fixed voltage io/vbus regulators to workaround
this issue. Fix the following warnings:

[    7.514572] xhci-mtk 1e1c0000.xhci: supply vbus not found, using dummy regulator
[    7.522375] xhci-mtk 1e1c0000.xhci: supply vusb33 not found, using dummy regulator

Signed-off-by: Shiji Yang <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/18886
Signed-off-by: Hauke Mehrtens <[email protected]>