x86 plat: limit x86 platform driver menu to X86

My .config contains ACER_WMI=m. On SPARC. That does not make sense.
Restrict the x86 platform driver menu to x86.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
Signed-off-by: Matthew Garrett <mjg@redhat.com>