ARM: rmobile: dts: Add XHCI USB node to r8a7796

The R8A7796 XHCI USB nodes are missing from r8a7796 dtsi, add them.
These nodes don't come from mainline Linux, yet the DT binding is
similar enough to R8A7795 which already has those in mainline and
once the nodes hit mainline, this DT should be resynched.

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