usb: xilinx: Add support for usb phy reset via gpio

Some USB phys on zynq boards needs to be reset before
usage. All boards are using reset through ps7_gpio
but another gpio controller can be used as well.
This patch adds support for gpio usb phy reset
which should be decribed in DTS in the format:

ps7_usb_0: ps7-usb@e0002000 {
	compatible = "xlnx,ps7-usb-1.00.a";
	usb-reset = <&ps7_gpio_0 7 0>;
	...
} ;

Signed-off-by: Michal Simek <michal.simek@xilinx.com>