ARM: dts: at91: sama5d2: add TCB GCLK

The sama5d2 tcbs take an extra input clock, their gclk.

Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200710230813.1005150-4-alexandre.belloni@bootlin.com