riscv: only enable ZONE_DMA32 for 64-bit

Until we actually support > 32bit physical addresses for 32-bit using
highmem there is no point in enabling ZONE_DMA32.  And even if such
support is ever added it probably should be conditional to not burden
low end embedded devices.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>