arm swiotlb fix

[Christoph Hellwig]

Hi all,

my commit to add swiotlb to arm failed to initialize the atomic pool,
which is needed for GFP_ATOMIC allocations on non-coherent devices.
These are fairly rare, but exist so we should wire it up.  For 5.4
I plan to move the initilization to the common dma-remap code so it
won't be missed for other architectures.