mfd: axp20x: name voltage ramping define properly

The current axp20x names the ramping register 'scal' which probably
means scaling. Since the register really has nothing to do with
scaling, but really is the voltage ramp we rename it appropriately.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Priit Laes <plaes@plaes.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>