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 <[e-mail]>
Reviewed-by: Palmer Dabbelt <[e-mail]>

arch/riscv/Kconfig

···
 
 config ZONE_DMA32
 	bool
-	default y
+	default y if 64BIT
 
 config PAGE_OFFSET
 	hex