ARM: dts: stm32: add pwm cells to stm32mp157c

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

Fixes: 61fc211c ("ARM: dts: stm32: add timers support to stm32mp157c")
Signed-off-by: Fabrice Gasnier <fabrice.gasnier@st.com>
Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>

arch/arm/boot/dts/stm32mp157c.dtsi

@@ -139,6 +139,7 @@ timers2: timer@40000000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -167,6 +168,7 @@ timers3: timer@40001000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -193,6 +195,7 @@ timers4: timer@40002000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -221,6 +224,7 @@ timers5: timer@40003000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -278,6 +282,7 @@ timers12: timer@40006000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -299,6 +304,7 @@ timers13: timer@40007000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -320,6 +326,7 @@ timers14: timer@40008000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -573,6 +580,7 @@ timers1: timer@44000000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -603,6 +611,7 @@ timers8: timer@44001000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -676,6 +685,7 @@ timers15: timer@44006000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 
@@ -700,6 +710,7 @@ timers16: timer@44007000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};
 			timer@15 {
@@ -723,6 +734,7 @@ timers17: timer@44008000 {
 
 			pwm {
 				compatible = "st,stm32-pwm";
+				#pwm-cells = <3>;
 				status = "disabled";
 			};