usb: xhci: octeon: Add DWC3 glue layer for Octeon

This patch adds the glue layer for the MIPS Octeon SoCs. It's ported
mainly from the Linux code.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Marek Vasut <marex@denx.de>