Lantiq SoC Serial To Parallel (STP) GPIO controller

The Serial To Parallel (STP) is found on MIPS based Lantiq socs. It is a
peripheral controller used to drive external shift register cascades. At most
3 groups of 8 bits can be driven. The hardware is able to allow the DSL modem
to drive the 2 LSBs of the cascade automatically.


Required properties:
- compatible : Should be "lantiq,gpio-stp-xway"
- reg : Address and length of the register set for the device
- #gpio-cells : Should be two.  The first cell is the pin number and
  the second cell is used to specify optional parameters (currently
  unused).
- gpio-controller : Marks the device node as a gpio controller.

Optional properties:
- lantiq,shadow : The default value that we shall assume as already set on the
  shift register cascade.
- lantiq,groups : Set the 3 bit mask to select which of the 3 groups are enabled
  in the shift register cascade.
- lantiq,dsl : The dsl core can control the 2 LSBs of the gpio cascade. This 2 bit
  property can enable this feature.
- lantiq,phy1 : The gphy1 core can control 3 bits of the gpio cascade.
- lantiq,phy2 : The gphy2 core can control 3 bits of the gpio cascade.
- lantiq,rising : use rising instead of falling edge for the shift register

Example:

gpio1: stp@e100bb0 {
	compatible = "lantiq,gpio-stp-xway";
	reg = <0xE100BB0 0x40>;
	#gpio-cells = <2>;
	gpio-controller;

	lantiq,shadow = <0xffff>;
	lantiq,groups = <0x7>;
	lantiq,dsl = <0x3>;
	lantiq,phy1 = <0x7>;
	lantiq,phy2 = <0x7>;
	/* lantiq,rising; */
};