phy: sun4i-usb: Add a sunxi specific function for setting squelch-detect

The sunxi otg phy has a bug where it wrongly detects a high speed squelch
when reset on the root port gets de-asserted with a lo-speed device.

The workaround for this is to disable squelch detect before de-asserting
reset, and re-enabling it after the reset de-assert is done. Add a sunxi
specific phy function to allow the sunxi-musb glue to do this.

Acked-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>