PCI: mvebu: Restrict build to 32-bit ARM

This driver uses PCI glue that is only available on 32-bit ARM.  This used
to work fine as long as ARCH_MVEBU and ARCH_DOVE were exclusively 32-bit,
but there's a patch in the pipe to make ARCH_MVEBU also available on 64-bit
ARM.

[bhelgaas: changelog; patch is coming but not merged yet]
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>