phy: Add USB PHY driver for the MT76x8 (7628/7688) SoC

This driver is derived from this Linux driver:
linux/drivers/phy/ralink/phy-ralink-usb.c

The driver sets up power and host mode, but also needs to configure PHY
registers for the MT7628 and MT7688.

I removed the reset controller handling for the USB host and device, as
it does not seem to be necessary right now. The soft reset bits for both
devices are enabled by default and testing has shown (with hackish
reset handling added), that USB related commands work identical with
or without the reset handling.

Please note that the resulting USB support is tested only very minimal.
I was able to detect one of my 3 currently available USB sticks.
Perhaps some further work is needed to fully support the EHCI controller
integrated in the MT76x8 SoC.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Marek Vasut <marex@denx.de>
Cc: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>