usb: ehci: Add Renesas RCar M3/H3 EHCI support

Add a USB controller driver for the EHCI block in R8A7795/R8A7796 SoC.
This is a stopgap measure until we have proper DT support, clock and
reset framework in place, at which point we can switch to ehci-generic.

Signed-off-by: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Hiroyuki Yokoyama <hiroyuki.yokoyama.vx@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Reviewed-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>