nios2: remove unneeded HAS_DMA define

kernel/dma/Kconfig globally defines HAS_DMA as follows:

  config HAS_DMA
          bool
          depends on !NO_DMA
          default y

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>