serial: 8250_dw: Use ifdef with ACPI

There are no stubs for ACPI functions so the driver needs to
have this ifdef or it will not compile without ACPI.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>