ACPI / PM: Make suspend_nvs_save() use acpi_os_map_memory()

It turns out that the NVS memory region that suspend_nvs_save()
attempts to map has been already mapped by acpi_os_map_memory(), so
suspend_nvs_save() should better use acpi_os_map_memory() for mapping
memory to avoid conflicts.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Len Brown <len.brown@intel.com>