mips: ath79: Add support for ungating USB on ar933x and ar934x

Add code to ungate the USB controller on ar933x and ar934x .

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Cc: Wills Wang <wills.wang@live.com>