ARM: dts: BCM5301X: Add CPU thermal sensor and zone

This uses CPU thermal sensor available on every Northstar chipset to
monitor temperature. We don't have any cooling or throttling so only a
critical trip was added.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Acked-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>