avr32: favr-32: use generic pwm_bl driver

Switch to the generic pwm_bl driver instead of atmel-pwm-bl.

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