drivers: led: pwm: automatically select PWM

For all LED drivers, the underlying subsystem is automatically selected
in Kconfig if their compatible node is found in DT.

The only exception is the PWM LED driver which depends on PWM instead
of selecting it. The PWM Kconfig option must be explicitely selected in
order to have LED_PWM enabled.

This patch updates the Kconfig of the PWM LED driver to have the same
behavior as other LED drivers: PWM is now automatically selected if a
"pwm-leds" compatible node is found in DT.

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