pc: acpi: factor out memhp code from build_ssdt() into separate function

pc: acpi: factor out memhp code from build_ssdt() into separate function

before consolidating memhp code in memory_hotplug_acpi_table.c
and for simplifying review, first factor out memhp code into
new function build_memory_devices() in i386/acpi-build.c

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
----
PS:
   no functional change, only code movement.
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>