ACPI: reboot: Avoid racing after writing to ACPI RESET_REG

According to the ACPI spec, "The system must reset immediately following
the write to the ACPI RESET_REG register.", but there are cases that the
system does not follow this and results in racing with the subsequetial
reboot mechanism, which brings unexpected behavior.

Fix this by adding a 15ms delay after writing to the ACPI RESET_REG.

Reported-by: Ghorai, Sukumar <sukumar.ghorai@intel.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>
[ rjw: Edit comment in the code and subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>