dt: bindings: add thermal device driver for bcm2835

Add dt-binding documentation for bcm2835 SOC thermal sensor.

Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Acked-by: Eric Anholt <eric@anholt.net>
Acked-by: Rob Herring <robh@kernel.org>

Changelog:
 V1 -> V2: renamed file to follow naming conventions
 V2 -> V3: removed 0x in node name
Signed-off-by: Eric Anholt <eric@anholt.net>