usb: host: xhci: plat: add support for Renesas r8a7796 SoC

This patch adds support for Renesas r8a7796 SoC. This SoC is not
compatible with r8a7795 because using firmware version differs.

Since the "V2" firmware can be used on both r8a7795 (es1.x) and r8a7796,
the "renesas,rcar-gen3-xhci" keeps to use the "V2" for now.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>