xtensa: remove ZONE_DMA

ZONE_DMA is intended for magic < 32-bit pools (usually ISA DMA), which
isn't required on xtensa.  Move all the non-highmem memory into
ZONE_NORMAL instead to match other architectures.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>