leds: atmel-pwm: remove obsolete driver

The leds-atmel-pwm driver is now obsolete. It is not used by any mainlined
boards and is replaced by the generic leds_pwm with the pwm-atmel driver using
the generic PWM framework.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>