usb: chipidea: imx: Do not access CLKONOFF on i.MX51

Unlike i.MX53, i.MX51's USBOH3 register file does not implemenent
registers past offset 0x018, which includes
MX53_USB_CLKONOFF_CTRL_OFFSET and trying to access that register on
said platform results in external abort.

Fix it by enabling CLKONOFF accessing codepath only for i.MX53.

Cc: stable <stable@vger.kernel.org>
Fixes 3be3251d ("usb: chipidea: imx: Disable internal 60Mhz
	clock with ULPI PHY")
Cc: cphealy@gmail.com
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Peter Chen <peter.chen@nxp.com>