thermal: brcmstb_thermal: Restructure interrupt registration

If we are successful grabbing the interrupt resource, then register an
interrupt handler, this makes it easier to support the interrupt as
being optional, which is it for 7216.

Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200114190607.29339-6-f.fainelli@gmail.com