x86: default to SWIOTLB=y on x86_64

Impact: fixes korg bugzilla 11980

A kernel for a 64bit x86 system should always contain the swiotlb code
in case it is booted on a machine without any hardware IOMMU supported
by the kernel and more than 4GB of RAM. This patch changes Kconfig to
always compile swiotlb into the kernel for x86_64.

Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>
Cc: stable@kernel.org
Signed-off-by: Ingo Molnar <mingo@elte.hu>