ACPI: reboot: Unify the message printing

The meesage printing in this file is mixed with pr_*() and
printk() but with no prefix and no pr_fmt() defined.

Intoduce pr_fmt() and use pr_*() macros to replace printk(),
to generate a unified format string for prefix.

Signed-off-by: Hanjun Guo <guohanjun@huawei.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>