regulator: pca9450: Use devm_register_sys_off_handler

regulator: pca9450: Use devm_register_sys_off_handler

With module test, there is error dump:
------------[ cut here ]------------
  notifier callback pca9450_i2c_restart_handler already registered
  WARNING: kernel/notifier.c:23 at notifier_chain_register+0x5c/0x88,
  CPU#0: kworker/u16:3/50
  Call trace:
  notifier_chain_register+0x5c/0x88 (P)
  atomic_notifier_chain_register+0x30/0x58
  register_restart_handler+0x1c/0x28
  pca9450_i2c_probe+0x418/0x538
  i2c_device_probe+0x220/0x3d0
  really_probe+0x114/0x410
  __driver_probe_device+0xa0/0x150
  driver_probe_device+0x40/0x114
  __device_attach_driver+0xd4/0x12c

So use devm_register_sys_off_handler to let kernel handle the resource
free to avoid kernel dump.

Fixes: 6157e62b07d9 ("regulator: pca9450: Add restart handler")
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://patch.msgid.link/20250815-pca9450-v1-1-7748e362dc97@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>