mfd: intel_pmt: Only compile on x86

The intel_pmt driver shows up as a compile option for all arches but is
32-bit and 64-bit x86 specific.

Add a CONFIG dependency on X86 for intel_pmt.

Signed-off-by: Prarit Bhargava <prarit@redhat.com>
Reviewed-by: David E. Box <david.e.box@linux.intel.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>