clocksource/drivers/timer-atmel-pit: Drop at91sam926x_pit_common_init

Merge at91sam926x_pit_common_init in at91sam926x_pit_dt_init as this is the
only initialization method now.

Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>