thermal: bcm2835: enable hwmon explicitly

By defaul of-based thermal driver do not enable hwmon.
This patch does this explicitly, so that the temperature can be read
through the common hwmon sysfs.

Signed-off-by: Matthias Brugger <mbrugger@suse.com>
Acked-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>