thermal: introduce THERMAL_MAX_LIMIT macro

Introduce THERMAL_MAX_LIMIT macro for thermal devicetree definition.
This macro will represent the maximum possible mitigation a cooling
device can mitigate to.

Change-Id: I2f5df4009524b42a442f11126c97b789187202c4
Signed-off-by: Ram Chandrasekar <rkumbako@codeaurora.org>