DWC3 host support

[Angus Ainslie]

Hi Michal,

I recently rebased my librem5 tree onto the latest u-boot-imx branch and 
the dwc3 host mode stopped working.

I bisected it down to this commit:

142d50fbce7c364a74f5e8204dba491b9f066e6c usb: dwc3: Add support for 
usb3-phy PHY configuration

Reverting that commit allows usb host mode to work on the librem5 again.

Should this initialization go into an SOC specific glue_configure 
function ?

Is the imx8mq.dtsi missing something that will keep usb host working 
with this patch ?

Does this break usb host on other imx8mq devices ?

Thanks
Angus