pwm: atmel-tcb: Add sama5d2 support

Add sama5d2 support. The sama5d2 has a new clock input, its gclk. Index 0
of the clock selector is the gclk instead of the peripheral clock divided
by 2.

For now, the gclk is not used because the peripheral clock divided by 8
already gives a 9.6ns resolution which is enough for most use cases.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>