MIPS: Netlogic: Fix DMA zone selection for 64-bit

Fix Kconfig for both XLR and XLP to select ZONE_DMA32 (instead of ZONE_DMA)
in case of 64-bit compilation. This can be used for devices that can only
do DMA to 32-bit address. ZONE_DMA is not useful on XLR or XLP.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4466
Signed-off-by: John Crispin <blogic@openwrt.org>