xen: SWIOTLB is only used on x86

Enabling SWIOTLB_XEN on ARM results in build errors because the
underlying SWIOTLB is only available on X86:

drivers/xen/swiotlb-xen.c: In function 'is_xen_swiotlb_buffer':
drivers/xen/swiotlb-xen.c:105:2: error: implicit declaration of function 'mfn_to_local_pfn

Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>