regulator: max8973: add support for junction thermal warning

The driver MAX8973 supports the driver for Maxim PMIC 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.

MAX77621 does not support the continuous temp monitoring of
junction temperature. It just report whether junction temperature
crossed the threshold or not.

Add support to
- Configure junction temp warning threshold via DT property
to generate alert when it crosses the threshold.
- Add support to interrupt the host from this device when alert
occurred.
- read the junction temp via thermal framework.

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