usb: phy: add run-time dependencies to R-Car driver

The Renesas R-Car USB PHY driver only supports the R8A7778 and
R8A7779, it isn't useful on other systems unless build testing.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>