boards: arm: gd32f403z_eval: enable pwm support

Enable timer0 and its pwm node. The board does not directly have any LED
connected to a PWM output. However, following the official
"TIMER_Breath_LED" example, user can bridge PA8 (TIMER0_CH0) to PF0
(LED2) to run quick samples such as samples/basic/blinky_pwm.

Note that a prescaler value of 256 has been chosen so that basic PWM
samples (blinky_pwm and fade_led) can be run. This value may be
overriden in specific samples if needed.

Signed-off-by: Gerard Marull-Paretas <gerard@teslabs.com>

boards/arm/gd32f403z_eval/doc/index.rst

@@ -66,6 +66,8 @@ The board configuration supports the following hardware features:
 | NVIC      | on-chip    | nested vectored       |
 |           |            | interrupt controller  |
 +-----------+------------+-----------------------+
+| PWM       | on-chip    | PWM                   |
++-----------+------------+-----------------------+
 | SYSTICK   | on-chip    | system clock          |
 +-----------+------------+-----------------------+
 | UART      | on-chip    | serial port-polling   |

boards/arm/gd32f403z_eval/gd32f403z_eval-pinctrl.dtsi

@@ -11,4 +11,10 @@
 			pinmux = <USART0_TX_PA9_NORMP>, <USART0_RX_PA10_NORMP>;
 		};
 	};
+
+	pwm0_default: pwm0_default {
+		group1 {
+			pinmux = <TIMER0_CH0_PA8_OUT_NORMP>;
+		};
+	};
 };

boards/arm/gd32f403z_eval/gd32f403z_eval.dts

@@ -59,10 +59,20 @@
 		};
 	};
 
+	pwmleds {
+		compatible = "pwm-leds";
+
+		/* NOTE: bridge TIMER0_CH0 (PA8) and LED2 (PF0) */
+		pwm_led: pwm_led {
+			pwms = <&pwm0 0 PWM_POLARITY_NORMAL>;
+		};
+	};
+
 	aliases {
 		led0 = &led2;
 		led1 = &led3;
 		sw0 = &user_key1;
+		pwm-led0 = &pwm_led;
 	};
 };
 
@@ -84,3 +94,14 @@
 	pinctrl-0 = <&usart0_default>;
 	pinctrl-names = "default";
 };
+
+&timer0 {
+	status = "okay";
+	prescaler = <256>;
+
+	pwm0: pwm {
+		status = "okay";
+		pinctrl-0 = <&pwm0_default>;
+		pinctrl-names = "default";
+	};
+};

boards/arm/gd32f403z_eval/gd32f403z_eval.yaml

@@ -11,3 +11,5 @@ toolchain:
   - zephyr
   - gnuarmemb
   - xtools
+supported:
+  - pwm