Add the platform-specific code for enabling SWIOTLB if needed on P2020DS,
MPC85xx DS, and MPC85xx MDS boards as they are capable of having >4G of
memory.

We determine if we need to enable swiotlb based on how much memory is in
the board and if it exceeds 4G or what we can map via PCI inbound
windows.
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>