regulator: max77620: Add details of device specific ramp rate setting

It is observed that the ramp rate measured on platform is not same as
advertised ramp rate due to platform design variation. On this case,
measured ramp rate is provided with property regulator-ramp-rate.

For such cases, add DT property for device specific configurations.

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