hw/usb: Add basic i.MX USB Phy support

Add basic USB PHY support as implemented in i.MX23, i.MX28, i.MX6,
and i.MX7 SoCs.

The only support really needed - at least to boot Linux - is support
for soft reset, which needs to reset various registers to their initial
value. Otherwise, just record register values.

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Message-id: 20200313014551.12554-2-linux@roeck-us.net
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>