thermal: max77620: Add DT binding doc for thermal driver

Maxim Semiconductor MAX77620 supports alarm interrupts when
its die temperature crosses 120C and 140C. These threshold
temperatures are not configurable.

Add DT binding document to details out the DT property related
to MAX77620 thermal functionality.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>