regulator: max8973: configure ramp delay through callback

Regulator core framework support the configuration of ramp
delay reading from platform specific regulator data via the
regulator callback ops.

Instead of reading regulator init data on driver and setting
ramp delay, use the callback to achieve this.

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