regulator: bd718x7, bd71828: Use ramp-delay helper

Use generic regamp ramp-delay helper function instead of implementing own.

Signed-off-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Acked-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>