kernel/reboot.c: add devm_register_reboot_notifier()

Add devm_* wrapper around register_reboot_notifier to simplify device
specific reboot notifier registration/unregistration.

Link: http://lkml.kernel.org/r/20170320171753.1705-1-andrew.smirnov@gmail.com


Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>