Revert "arm64: Revert L1_CACHE_SHIFT back to 6 (64-byte cache line size)"

Revert "arm64: Revert L1_CACHE_SHIFT back to 6 (64-byte cache line size)"

This reverts commit 33a72f907319bfef50b212309bcd898a465c06dc.

The internal dma-direct.h API has changed in -next, which collides with
us trying to use it to manage non-coherent DMA devices on systems with
unreasonably large cache writeback granules.

This isn't at all trivial to resolve, so revert our changes for now and
we can revisit this after the merge window. Effectively, this just
restores our behaviour back to that of 4.16.

Signed-off-by: Will Deacon <will.deacon@arm.com>