mfd: axp20x: Add support for AXP717 PMIC

The AXP717a is a PMIC chip produced by X-Powers, it can be connected to
an I2C or RSB bus.

It's a rather complete PMIC, with many regulators, interrupts, an ADC and
battery charging functionality. It also offer USB type-C CC pin
handling.

Describe the regmap and the MFD bits, along with the registers exposed
via I2C or RSB. This covers the regulator, interrupts and power key
devices for now.
Advertise the device using the new compatible string.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Tested-by: Ryan Walklin <ryan@testtoast.com>
Link: https://lore.kernel.org/r/20240310010211.28653-4-andre.przywara@arm.com


Signed-off-by: Lee Jones <lee@kernel.org>