thermal: armada: convert driver to syscon register accesses

Until recently, only one register was referenced in MVEBU thermal IP
node. Recent changes added a second entry pointing to another
register right next to it. We cannot know for sure that we will not
have to access other registers. That will be actually the case when
overheat interrupt feature will come, where it will be needed to access
DFX registers in the same area.

This approach is not scalable so instead of adding consinuously memory
areas in the DT (and change the DT bindings, while keeping backward
compatibility), move the thermal node into a wider syscon from which it
will be possible to also configure the thermal interrupt.

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