usb: ehci: rmobile: Add support R8A7793

R8A7793 has same IP of USB controller as R8A7791 and R8A7794 of rmobile
ARM SoCs. This adds support R8A7793 to EHCI HCD of rmobile.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>