dt-bindings: mfd: st,stmpe: Add missing properties for PWM subnode

dt-bindings: mfd: st,stmpe: Add missing properties for PWM subnode

The st,stmpe-pwm binding is already covered by the MFD schema in
Documentation/devicetree/bindings/mfd/st,stmpe.yaml. However, the
PWM subnode was missing a 'required' properties block. This allowed
Device Tree nodes to pass validation even if the 'compatible'
string was omitted. This omission could lead to probe failures
at runtime.

Fix the schema by adding the missing 'required' block.

Signed-off-by: Manish Baing <manishbaing2789@gmail.com>
Acked-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Uwe Kleine-König <ukleinek@kernel.org>
Reviewed-by: Linus Walleij <linusw@kernel.org>
Link: https://patch.msgid.link/20260523173251.72540-2-manishbaing2789@gmail.com
Signed-off-by: Lee Jones <lee@kernel.org>