clocksource/drivers/timer-atmel-tcb: Add sama5d2 support

The first divisor for the sama5d2 is actually the gclk selector. Because
the currently remaining divisors are fitting the use case, currently ensure
it is skipped.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-10-alexandre.belloni@bootlin.com