backlight: atmel-pwm-bl: remove obsolete driver

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

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Hans-Christian Egtvedt <egtvedt@samfundet.no>
Acked-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>