usb: ehci: rmobile: Add support ehci host driver of rmobile SoCs

The rmobile SoC has usb host controller.
This supports USB controllers listed in the R8A7790, R8A7791 and R8A7740.

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>
Reviewed-by: Marek Vasut <marex@denx.de>