Include arch/acpi.h instead of manually adding acpi_slp_type.

Include arch/acpi.h instead of manually adding acpi_slp_type.

acpi_slp_type is defined in arch/acpi.h, so let's use that instead
of manually spreading extern u8 acpi_slp_type throughout the code.

Change-Id: Ia5eb420364c15ab5a764bc328bbd201ca9cb7837
Signed-off-by: Stefan Reinauer <reinauer@google.com>
Reviewed-on: http://review.coreboot.org/714
Tested-by: build bot (Jenkins)
Reviewed-by: Ronald G. Minnich <rminnich@gmail.com>