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>