usb: misc: Add driver for usb4604

This is a minimal driver to support bringing a usb4604 device
from microchip out of reset and into hub mode. The usb4604 device
is related to the usb3503 device, but it didn't seem close enough
to warrant putting both into the same file. This patch borrows
some of the usb3503 structure and trims it down to just handle
the optional reset gpio and adds the i2c command to put the
device into hub mode.

Datasheet: http://ww1.microchip.com/downloads/en/DeviceDoc/00001716A.pdf
Cc: <devicetree@vger.kernel.org>
Signed-off-by: Stephen Boyd <stephen.boyd@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/usb/usb4604.txt

@@ -0,0 +1,19 @@
+SMSC USB4604 High-Speed Hub Controller
+
+Required properties:
+- compatible: Should be "smsc,usb4604"
+
+Optional properties:
+- reg: Specifies the i2c slave address, it is required and should be 0x2d
+       if I2C is used.
+- reset-gpios: Should specify GPIO for reset.
+- initial-mode: Should specify initial mode.
+                (1 for HUB mode, 2 for STANDBY mode)
+
+Examples:
+	usb-hub@2d {
+		compatible = "smsc,usb4604";
+		reg = <0x2d>;
+		reset-gpios = <&gpx3 5 1>;
+		initial-mode = <1>;
+	};