pwm: atmel-tcb: Put per-channel data into driver data

This simplifies the code, reduces the number of memory allocations and
pointer dereferences.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>