thermal: imx: register irq handler later in probe

The irq handler should be registered after the tempmon
module has been initialized in a known state and the
thermal_zone and cpu_cooling device have been registered
successfully. Otherwise, if the irq is triggled earlier
before thermal probe has been finished, it may lead to
'NULL' pointer kernel panic.

Signed-off-by: Bai Ping <b51503@freescale.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>