regulator: palmas-pmic: doc: remove ti,tstep

commit 28d1e8cd
(regulator: palma: add ramp delay support through regulator constraints)

Removed the regulator's ti,step option from driver without updating the
documentation.

So, remove from documentation and example as well.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mark Brown <broonie@linaro.org>