phy: rcar: Add R-Car Gen2 PHY driver

Add a PHY driver for the R-Car Gen2 which allows configuring the mux
connected to the EHCI controllers and USBHS controller.

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