MIPS: Malta: Select 32bit DMA zone for 64-bit kernels

MIPS: Malta: Select 32bit DMA zone for 64-bit kernels

Enable the 32-bit DMA zone for 64-bit Malta kernels so that devices with
32-bit coherent DMA masks aren't constrained to the low 16MB DMA zone,
which can easily be exhausted when there is lots of static kernel data
due to lock and RCU debugging.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9890/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>