usb: doc: phy-mxs: Add more compatible strings

Add "fsl,imx6q-usbphy" for imx6dq and imx6dl, add
"fsl,imx6sl-usbphy" for imx6sl, and "fsl,imx23-usbphy"
is still a fallback for other strings.
Signed-off-by: Peter Chen <peter.chen@freescale.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>

Documentation/devicetree/bindings/usb/mxs-phy.txt

 * Freescale MXS USB Phy Device
 
 Required properties:
-- compatible: Should be "fsl,imx23-usbphy"
+- compatible: should contain:
+	* "fsl,imx23-usbphy" for imx23 and imx28
+	* "fsl,imx6q-usbphy" for imx6dq and imx6dl
+	* "fsl,imx6sl-usbphy" for imx6sl
+  "fsl,imx23-usbphy" is still a fallback for other strings
 - reg: Should contain registers location and length
 - interrupts: Should contain phy interrupt