pwm: atmel: Drop unused header

Since commit 52eaba4c ("pwm: atmel: Rework tracking updates pending
in hardware") the driver doesn't make use of mutexes any more, so the
header defining these doesn't need to be included.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>