mfd: axp20x: Fix axp288 writable-ranges

Register AXP288_POWER_REASON is writable and needs to be written
to reset the reset- / power-on-reason bits.

Add it to the axp288 writable-ranges so that the extcon-axp288
driver can properly clear the reset- / power-on-reason bits.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230329205544.1051393-1-hdegoede@redhat.com