staging: gma500: Depend on X86

 The gma500 driver calls set_pages_uc, which is an x86 pageattr call.
 Since this driver is only used with Intel x86 motherboard chipsets,
 make the driver depend on X86.

Signed-off-by: Jeff Mahoney <jeffm@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>