nds32: introduce DMA allocation API

U-Boot does not compile for the adp-ag101 boards since
commit a8f9cd18
(net: update FTGMAC100 for MMU/D-cache support)

The driver assumes that the DMA allocation API are provided by all
architectures. This is not the case for nds32 and it causes a
build error. This patch adds DMA allocation API to avoid the errors.

Signed-off-by: Kuan-Yu Kuo <ken.kuoky@gmail.com>
Cc: Macpaul Lin <macpaul@gmail.com>
Cc: Andes <uboot@andestech.com>
Signed-off-by: Andes <uboot@andestech.com>