usb: xhci: Add Renesas R-Car xHCI driver

Add firmware V3, firmware loader and XHCI glue for the Renesas R-Car
Gen3 SoCs XHCI controller. Thus far only the R-Car Gen3 R8A7795 ES2.0+
and R8A7796 are supported.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Bin Meng <bmeng.cn@gmail.com>