pwm: atmel-tcb: Make atmel_tcb_divisors static

The array atmel_tcb_divisors is not supposed to be used outside of the
driver, so make it static.

This fixes a sparse warning:

	drivers/pwm/pwm-atmel-tcb.c:64:10: warning: symbol 'atmel_tcb_divisors' was not declared. Should it be static?

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