ACPI: scan: substitute empty_zero_page with helper ZERO_PAGE(0)

Not all zero page implementations use empty_zero_page global pointer so
let's substitute empty_zero_page occurence with helper ZERO_PAGE(0).

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>