dts/bindings: Make pwm-leds label optional

The label property in the pwm-leds sub-node is optional, so mark it as
such.

Fixes: zephyrproject-rtos#17665

Signed-off-by: Kumar Gala <[email protected]>

Author: galak

dts/bindings/led/pwm-leds.yaml

@@ -23,6 +23,6 @@ sub-node:
           category: required
 
         label:
-          category: required
+          category: optional
           type: string
           description: Human readable string describing the device (used by Zephyr for API name)