mfd: axp20x: Allow multiple regulators

At the moment trying to register a second AXP chip makes the probe fail,
as some sysfs registration fails due to a duplicate name:

...
[ 3.688215] axp20x-i2c 0-0035: AXP20X driver loaded
[ 3.695610] axp20x-i2c 0-0036: AXP20x variant AXP323 found
[ 3.706151] sysfs: cannot create duplicate filename '/bus/platform/devices/axp20x-regulator'
[ 3.714718] CPU: 0 UID: 0 PID: 1 Comm: swapper/0 Not tainted 6.12.0-rc1-00026-g50bf2e2c079d-dirty #192
[ 3.724020] Hardware name: Avaota A1 (DT)
[ 3.728029] Call trace:
[ 3.730477] dump_backtrace+0x94/0xec
[ 3.734146] show_stack+0x18/0x24
[ 3.737462] dump_stack_lvl+0x80/0xf4
[ 3.741128] dump_stack+0x18/0x24
[ 3.744444] sysfs_warn_dup+0x64/0x80
[ 3.748109] sysfs_do_create_link_sd+0xf0/0xf8
[ 3.752553] sysfs_create_link+0x20/0x40
[ 3.756476] bus_add_device+0x64/0x104
[ 3.760229] device_add+0x310/0x760
[ 3.763717] platform_device_add+0x10c/0x238
[ 3.767990] mfd_add_device+0x4ec/0x5c8
[ 3.771829] mfd_add_devices+0x88/0x11c
[ 3.775666] axp20x_device_probe+0x70/0x184
[ 3.779851] axp20x_i2c_probe+0x9c/0xd8
...

This is because we use PLATFORM_DEVID_NONE for the mfd_add_devices()
call, which would number the child devices in the same 0-based way, even
for the second (or any other) instance.

Use PLATFORM_DEVID_AUTO instead, which automatically assigns
non-conflicting device numbers.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20241007001408.27249-4-andre.przywara@arm.com
Signed-off-by: Lee Jones <lee@kernel.org>