ARM: dts: bcm283x: Add CPU thermal zone with 1 trip point

As suggested by Eduardo Valentin this adds the thermal zone for
the bcm2835 SoC with its single thermal sensor. We start with
the criticial trip point and leave the cooling devices empty
since we don't have any at the moment. Since the coefficients
could vary depending on the SoC we need to define them separate.

Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Signed-off-by: Eric Anholt <eric@anholt.net>
Acked-by: Eduardo Valentin <edubezval@gmail.com>