thermal: broadcom: Use thermal_zone_get_crit_temp() in bcm2835_thermal_probe()

Modify the bcm2835 thermal driver to use thermal_zone_get_crit_temp() in
bcm2835_thermal_probe() instead of relying on the assumption that the
critical trip index will always be 0.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://patch.msgid.link/3322893.aeNJFYEL58@rjwysocki.net