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 torvalds#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>

drivers/mfd/axp20x.c

@@ -1419,7 +1419,7 @@ int axp20x_device_probe(struct axp20x_dev *axp20x)
 		}
 	}
 
-	ret = mfd_add_devices(axp20x->dev, -1, axp20x->cells,
+	ret = mfd_add_devices(axp20x->dev, PLATFORM_DEVID_AUTO, axp20x->cells,
 			      axp20x->nr_cells, NULL, 0, NULL);
 
 	if (ret) {