thermal: armada: get rid of the ->is_valid() pointer

The implementation of armada_is_valid() is very simple and is the same
across all the versions of the IP since the ->is_valid_bit has been
introduced. Simplify the structure by getting rid of the function
pointer and calling directly the function.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>