ARM: dts: stm32: add pwm cells to stm32f746

STM32 Timers support generic 3 cells PWM to encode PWM number, period and
polarity.

Fixes: 9bd7b77a ("ARM: dts: stm32: add Timers driver for stm32f746
MCU")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32f746.dtsi

@@ -94,6 +94,7 @@ timers2: timers@40000000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -123,6 +124,7 @@ timers3: timers@40000400 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -152,6 +154,7 @@ timers4: timers@40000800 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -180,6 +183,7 @@ timers5: timers@40000c00 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -249,6 +253,7 @@ timers12: timers@40001800 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -270,6 +275,7 @@ timers13: timers@40001c00 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 		};
@@ -285,6 +291,7 @@ timers14: timers@40002000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 		};
@@ -419,6 +426,7 @@ timers1: timers@40010000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -440,6 +448,7 @@ timers8: timers@40010400 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -512,6 +521,7 @@ timers9: timers@40014000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -533,6 +543,7 @@ timers10: timers@40014400 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 		};
@@ -548,6 +559,7 @@ timers11: timers@40014800 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 		};