ARM: dts: r8a7743: Add xhci support to SoC dtsi

Add node for xhci. Boards DT files will enable it if needed.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>