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>

arch/riscv/Kconfig

@@ -48,7 +48,7 @@ config MMU
 
 config ZONE_DMA32
 	bool
-	default y
+	default y if 64BIT
 
 config PAGE_OFFSET
 	hex