ARM: dts: am57xx-beagle-x15: Add thermal map to include fan and tmp102

BeagleBoard-X15 has capability for a fan and has an onboard TMP102
temperature sensor as well. This allows us to create a new thermal
zone (called, un-imaginatively "board"), and allows us to use some
active cooling as temperatures start edge upward in the system by
creating a new alert temperature (emperically 50C) for cpu.

NOTE: Fan is NOT mounted by default on the platform, in such a case,
all we end up doing is switch on a regulator and leak very minimal
current.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>