m68knommu: set NO_DMA

m68knommu does not set the Kconfig NO_DMA variable, but also does
not provide the required functions, resulting in the following
build error triggered by commit a40c24a1
(net: Add SKB DMA mapping helper functions.):

<--  snip  -->

..
  LD      vmlinux
net/built-in.o: In function `skb_dma_unmap':
(.text+0xac5e): undefined reference to `dma_unmap_single'
net/built-in.o: In function `skb_dma_unmap':
(.text+0xac7a): undefined reference to `dma_unmap_page'
net/built-in.o: In function `skb_dma_map':
(.text+0xacdc): undefined reference to `dma_map_single'
net/built-in.o: In function `skb_dma_map':
(.text+0xace8): undefined reference to `dma_mapping_error'
net/built-in.o: In function `skb_dma_map':
(.text+0xad10): undefined reference to `dma_map_page'
net/built-in.o: In function `skb_dma_map':
(.text+0xad82): undefined reference to `dma_unmap_page'
net/built-in.o: In function `skb_dma_map':
(.text+0xadc6): undefined reference to `dma_unmap_single'
make[1]: *** [vmlinux] Error 1

<--  snip  -->

Signed-off-by: Adrian Bunk <bunk@kernel.org>
Signed-off-by: Greg Ungerer <gerg@uclinux.org>