mfd: axp20x: Ensure relationship between IDs and model names

At the moment there is an implicit relationship between the AXP model
IDs and the order of the strings in the axp20x_model_names[] array.
This is fragile, and makes adding IDs in the middle error prone.

Make this relationship official by changing the ID type to the actual
enum used, and using indexed initialisers for the string list.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20241007001408.27249-3-andre.przywara@arm.com


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