pinctrl: pinctrl-single: Make sure we do not change bits outside of mask

Use the pcs->fmask to make sure that the value is not changing (setting)
bits in areas where it should not.
To avoid situations like this:

pmx_dummy: pinmux@4a100040 {
	compatible = "pinctrl-single";
	reg = <0x4a100040 0x0196>;
	#address-cells = <1>;
	#size-cells = <0>;
	pinctrl-single,register-width = <16>;
	pinctrl-single,function-mask = <0x00ff>;
};

&pmx_dummy {
	pinctrl-names = "default";
	pinctrl-0 = <&board_pins>;

	board_pins: pinmux_board_pins {
		pinctrl-single,pins = <
			0x6c 0xf0f
			0x6e 0x10f
			0x70 0x23f
			0x72 0xa5f
		>;
	};
};

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>