dts: bindings: fix label properties for pwm-leds node

This patch adds a label property to the PWM LED parent node and fixes
the label property description of the child node. Indeed it is the label
of a parent node which can be used as device_get_binding() parameter to
retrieve the LED device, and not the one of a child node.

Fixes #34275

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>