power: axp209: Add support for voltage rate control on LDO3

power: axp209: Add support for voltage rate control on LDO3

The AXP209 LDO3 regulator supports voltage rate control, or can set a
slew rate.

This allows for the power to gradually rise up to the desired voltage,
instead of spiking up as fast as possible. Reason to have this can be
to reduce the inrush currents for example.

There are 3 slopes to choose from, the default, 'none' is a voltage rise
of 0.0167 V/uS, a 1.6 mV/uS and a 0.8 mV/uS voltage rise.

In ideal world (where vendors follow the recommended design guidelines)
this setting should not be enabled by default. Unless of course AXP209
crashes instead of reporting overcurrent condition as it normally should
do in this case.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
Signed-off-by: Priit Laes <plaes@plaes.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>