misc: atmel_pwm: remove obsolete driver

The misc/atmel_pwm is not used by any mainlined boards and has been replaced by
the pwm-driver using the generic PWM framework.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>