regulator: max8973: add DT binding details for junction warn temp

The driver MAX8973 supports the driver for Maxim MAX77621.
MAX77621 supports the junction temp warning at 120 degC and
140 degC which is configurable. It generates alert signal when
junction temperature crosses these threshold.

Add DT properties and its binding details to make this
configuration from DT.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@kernel.org>