pinctrl: pinconf-generic: Add API for pinmux propertity in DTS file

When describing pin mux func through pinmux propertity,
a standard API is added for support. The pinmux contains pin
identification and mux values, which can include multiple
pins. And groups configuration use other word. DTS such as:

func-name {
	group_alias: group-name{
		pinmux= <pin_id << 8 | mux_value)>,
			<pin_id << 8 | mux_value)>;
		bias-pull-up;
		drive-strength-microamp = <4000>;
	};
};

Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Link: https://lore.kernel.org/20250212-amlogic-pinctrl-v5-2-282bc2516804@amlogic.com


Signed-off-by: Linus Walleij <linus.walleij@linaro.org>