usb: xhci-rcar: Add support for R8A774A1 SoC

The R8A774A1 is compatible with the generic rcar-gen3-xhci controller.
This patch adds the compatibility flag, to support the xHCI controller.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Biju Das <biju.das.jz@bp.renesas.com>