pinctrl: berlin: as370: use generic "pwm" as pwm function name

So that we could use the generic "pwm" for two or more pins, e.g

	pwm0_pmux: pwm0-pmux {
		groups = "PWM0", "PWM1";
		function = "pwm";
	};

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>