usb: dwc2: add support for APM82181 USB OTG

adds the specific compatible string for the DWC2 IP found in the APM82181
SoCs. The IP is setup correctly through the auto detection... With the
exception of the AHB Burst Size. The default of GAHBCFG_HBSTLEN_INCR4 of
the "snps,dwc2" can cause a system hang when the USB and SATA is used
concurrently. Because the predecessor (PPC460EX (Canyonlands)) already
had the same problem, this SoC can make use of the existing
dwc2_set_amcc_params() function.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>