arm64: dts: mt8173: add reference clock for usb

Due to the reference clock comes from 26M oscillator directly
on mt8173, and it is a fixed-clock in DTS which always turned
on, we ignore it before. But on some platforms, it comes
from PLL, and need be controlled, so here add it, no matter
it is a fixed-clock or not.

Signed-off-by: Chunfeng Yun <chunfeng.yun@mediatek.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>