pwm: mediatek: add pwm3 support for mt7981

This patch adds pwm channel 2 (pwm3) support for mt7981

diff --git a/arch/arm/dts/mt7981-rfb.dts b/arch/arm/dts/mt7981-rfb.dts
index 9b61373..d6ebd65 100644
--- a/arch/arm/dts/mt7981-rfb.dts
+++ b/arch/arm/dts/mt7981-rfb.dts
@@ -123,6 +123,14 @@
 			groups = "pwm0_1", "pwm1_0", "pwm2";
 		};
 	};
+
+	/* pin13 as pwm0, pin15 as pwm1, pin7 as pwm2 */
+	three_pwm_pins_1: three-pwm-pins {
+		mux {
+			function = "pwm";
+			groups = "pwm0_0", "pwm1_1", "pwm2";
+		};
+	};
 };
 
 &spi0 {