ACPI / bus: Only call dmi_check_system() on X86

Calling dmi_check_system() early only works on X86. Other
architectures initialize the DMI subsystem later so it's not
ready yet when ACPI itself gets initialized.

In the best case it results in a useless call to a function which
will do nothing. But depending on the dmi implementation, it could
also result in warnings. Best is to not call the function when it
can't work and isn't needed.

Additionally, if anyone ever needs to add non-x86 quirks, it would
surprisingly not work, so document the limitation to avoid confusion.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: cce4f632 (ACPI: fix early DSDT dmi check warnings on ia64)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>