dt-bindings: usb-xhci: Add r8a77990 support

This patch adds r8a77990 xhci support. The driver will use
the fallback compatible string "renesas,rcar-gen3-xhci".

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>