pwm: atmel-tcb: Support backup mode

Save and restore registers for the PWM on suspend and resume, which
makes hibernation and backup modes possible.

Signed-off-by: Romain Izard <romain.izard.pro@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>