arm64: Work around Cortex-A510 erratum 2454944

Cortex-A510 erratum 2454944 may cause clean cache lines to be
erroneously written back to memory, breaking the assumptions we rely on
for non-coherent DMA. Try to mitigate this by implementing special DMA
ops that do their best to avoid cacheable aliases via a combination of
bounce-buffering and manipulating the linear map directly, to minimise
the chance of DMA-mapped pages being speculated back into caches.

The other main concern is initial entry, where cache lines covering the
kernel image might potentially become affected between being cleaned by
the bootloader and the kernel being called, so perform some additional
maintenance to be safe in that regard too. Cortex-A510 supports S2FWB,
so KVM should be unaffected.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>