dt-bindings: mfd: axp20x: add support for regulator-ramp-delay for AXP209

The AXP209 supports ramping up voltages on several regulators such as
DCDC2 and LDO3, therefore we can use the standard 'regulator-ramp-delay'
property for those 2 regulators.

Note that the voltage ramp only works when the regulator is already
enabled. E.g. when going from say 0.7 V to 3.6 V.

When turning on the regulator, no voltage ramp is performed in hardware.

What this means, is that if the bootloader brings up the voltage at 0.7 V,
the ramp delay property is properly applied. If however, the bootloader
leaves the power off, no ramp delay is applied when the power is
enabled by the regulator framework.

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