misc: atmel_tclib: Do not probe already used TCBs

The TCBs that have children are using the proper DT bindings and don't need
to be handled by tclib.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>