ACPI: fix early DSDT dmi check warnings on ia64

WARNING: at drivers/firmware/dmi_scan.c:423 dmi_matches+0x70/0x160()
dmi check: not initialized yet.

This is caused by commit aa2110cb
(ACPI: add boot option acpi=copy_dsdt to fix corrupt DSDT).
DMI is not initialized yet in acpi_early_init on ia64.

The DSDT DMI check table is x86 specific, so make it empty on other archs.
And this fixes the warnings on ia64.

Reported-and-tested-by: Tony Luck <tony.luck@intel.com>
Signed-off-by: Lin Ming <ming.m.lin@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>