avm_gpio-API using the gpiolib

This is a pseudo-device that takes an GPIO and allows its use via the avm_gpio
API.

Required properties:
- compatible:		"avm,avm_generic_gpio"
- gpio:			The GPIO to expose via the avm_gpio API.

Exactly one of these properties is required:
- avm,input:		Initialize the GPIO as an input.
- avm,output:		Initialize the GPIO as an output.

Optional properties:
- pinctrl-*:		pinctrl-Properties as usual. The driver will try to map
			the avm_gpio FUNCTION_PINMUX1, FUNCTION_PINMUX2 and
			FUNCTION_PINMUX3 to the the pinctrl-states "mux1",
			"mux2" and "mux3" accordingly.

Examples of avm_gpio nodes:
	gpio_avm_button_dect {
		compatible = "avm,avm_gpio_generic";
		gpio = <&gpio1 0 GPIO_ACTIVE_HIGH>;
		avm,input;
	};

	gpio_avm_pcmlink_dcl {
		compatible = "avm,avm_gpio_generic";
		gpio = <&gpio0 31 GPIO_ACTIVE_HIGH>;
		avm,output;
		pinctrl-names = "mux3";
		pinctrl-0 = <&pinctrl_tdm_dcl>;
	};