usb: otg: add basic mxs phy driver support

mxs phy is used in Freescale i.MX SoCs, for example
imx23, imx28, imx6Q. This patch adds the basic host
support.

Signed-off-by: Richard Zhao <richard.zhao@freescale.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Peter Chen <peter.chen@freescale.com>
Acked-by: Felipe Balbi <balbi@ti.com>
Tested-by: Subodh Nijsure <snijsure@grid-net.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>