regulator: max8973: add support to configure ETR based on rail load

Add support to configure Enhanced Transient Response Enable (ETR)
and Sensitivity Selection based on maximum current i.e. expected
load on that rail.

Maxim recommended as:
- Enable ETR with high sensitivity (75mV/us) for 0 to 9A expected loads,
- Enable ETR with low sensitivity (150mV/us) for 9A to 12A expected loads.
- Disable ETR for expected load > 12A.

These recommendation will be configured for MAX77621 when maximum load
is provided through regulator constraint for maximum current from platform.

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