dt-bindings: pwm: describe the cells in #pwm-cells in pwm.yaml

When commit 89650a1e3b6f ("dt-bindings: pwm: Convert PWM bindings
to json-schema") converted the pwm provider section of the text binding
to dt-schema it also updated all references to pwm.txt in pwm provider
bindings to pwm.yaml.

Most pwm provider bindings had a reference to pwm.txt as it contains a
description of what the cells in #pwm-cells are, albeit in the consumer
section of the document. Only information in the provider section of the
document was moved to the yaml binding, and it contains no information
about the cell format, making all references to it for the cell format
unhelpful.

Fixes: 89650a1e3b6f ("dt-bindings: pwm: Convert PWM bindings to json-schema")
Signed-off-by: Conor Dooley <[e-mail]>
Reviewed-by: Rob Herring (Arm) <[e-mail]>
Link: https://lore.kernel.org/r/20240517-patient-stingily-30611f73e792@spud
Signed-off-by: Uwe Kleine-König <[e-mail]>

Documentation/devicetree/bindings/pwm/pwm.yaml

···
     pattern: "^pwm(@.*|-([0-9]|[1-9][0-9]+))?$"
 
   "#pwm-cells":
-    description:
-      Number of cells in a PWM specifier.
+    description: |
+      Number of cells in a PWM specifier. Typically the cells represent, in
+      order: the chip-relative PWM number, the PWM period in nanoseconds and
+      optionally a number of flags (defined in <dt-bindings/pwm/pwm.h>).
 
 required:
   - "#pwm-cells"