x86/ACPI: fix mapping of FACS

x86/ACPI: fix mapping of FACS

acpi_fadt_parse_sleep_info() runs when the system is already in
SYS_STATE_boot. Hence its direct call to __acpi_map_table() won't work
anymore. This call should probably have been replaced long ago already,
as the layering violation hasn't been necessary for quite some time.

Fixes: 1c4aa69ca1e1 ("xen/acpi: Rework acpi_os_map_memory() and acpi_os_unmap_memory()")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Acked-by: Roger Pau Monné <roger.pau@citrix.com>