fix call to pwm_set: no need to cast pointer arg

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
arch-atmega/pwm.c: new functions init_timer0 and deinit_timer0

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>
arch-atmega/pwm.c: implemented handling of 16bits pwm's (OC1A, OC1B)

Signed-off-by: Aurelio Colosimo <aurelio@aureliocolosimo.it>