Altera GPIO controller bindings

Required properties:
- compatible:
  - "altr,pio-1.0"
- reg: Physical base address and length of the controller's registers.

Optional properties:
- altr,gpio-bank-width: Width of the GPIO bank. This defines how many pins the
  GPIO device has. Ranges between 1-32. Optional and defaults to 32 if not
  specified.
- gpio-bank-name: bank name attached to this device.

Example:

user_led_pio_8out: gpio@0x4cc0 {
	compatible = "altr,pio-1.0";
	reg = <0x00004cc0 0x00000010>;
	resetvalue = <255>;
	altr,gpio-bank-width = <8>;
	#gpio-cells = <2>;
	gpio-controller;
	gpio-bank-name = "led";
};

In this example, the gpio can be accessed as led[0..7] using gpio command of
u-boot.
==> gpio clear led0