dts: bindings: pwm-leds: add description for pwms property

This patch adds a description section for the pwms property of the PWM
LED child node. This intends to explain how the period field is used by
the led_pwm driver and to help with its configuration.

Reported-by: Scott Worley <scott.worley@microchip.com>
Signed-off-by: Simon Guinot <simon.guinot@seagate.com>