usb: dwc2: add amcc,dwc-otg support

This patch adds support for the "amcc,usb-otg" device
which is found in the PowerPC Canyonlands' dts.

The device definition was added by:

commit c89b3458 ("powerpc/44x: Add USB DWC DTS entry to Canyonlands
board")

but without any driver support as the dwc2 driver wasn't available at
that time.

Note: The system can't use the generic "snps,dwc2" compatible
because of the special ahbcfg configuration. The default
GAHBCFG_HBSTLEN_INCR4 of snps,dwc2 can cause a system hang
when the USB and SATA is used concurrently.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>