ARM: dts: rmobile: Add xHCI node to R8A77965 M3N DT

Add xHCI node to r8a77965.dtsi to get xHCI USB operational.

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