thermal: armada: rename the initialization routine

Calling a hook ->init_sensor() while what is initialized is the IP
itself and not the sensors is misleading. Rename the hook ->init() to
avoid any confusion in later work bringing multi-sensors support.

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