Spreadtrum GPIO controller bindings

The controller's registers are organized as sets of sixteen 16-bit
registers with each set controlling a bank of up to 16 pins. A single
interrupt is shared for all of the banks handled by the controller.

Required properties:
- compatible: Should be "sprd,sc9860-gpio".
- reg: Define the base and range of the I/O address space containing
the GPIO controller registers.
- gpio-controller: Marks the device node as a GPIO controller.
- #gpio-cells: Should be <2>. The first cell is the gpio number and
the second cell is used to specify optional parameters.
- interrupt-controller: Marks the device node as an interrupt controller.
- #interrupt-cells: Should be <2>. Specifies the number of cells needed
to encode interrupt source.
- interrupts: Should be the port interrupt shared by all the gpios.

Example:
	ap_gpio: gpio@40280000 {
		compatible = "sprd,sc9860-gpio";
		reg = <0 0x40280000 0 0x1000>;
		gpio-controller;
		#gpio-cells = <2>;
		interrupt-controller;
		#interrupt-cells = <2>;
		interrupts = <GIC_SPI 50 IRQ_TYPE_LEVEL_HIGH>;
	};