ARM: dts: rmobile: Add EHCI nodes to R8A77965 M3N DT

Add generic EHCI nodes to r8a77965.dtsi to get EHCI USB operational.

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