MIPS: Alchemy: fix deprecated compile warnings

Replace deprecated DMA_32BIT_MASK with DMA_BIT_MASK.  This is needed because
the new style platform makefiles will enable -Werror.

cc1: warnings being treated as errors
arch/mips/alchemy/devboards/db1200/platform.c:219: error: 'DMA_nnBIT_MASK' is deprecated
arch/mips/alchemy/devboards/db1200/platform.c:226: error: 'DMA_nnBIT_MASK' is deprecated
arch/mips/alchemy/devboards/db1200/platform.c:388: error: 'DMA_nnBIT_MASK' is deprecated
arch/mips/alchemy/devboards/db1200/platform.c:393: error: 'DMA_nnBIT_MASK' is deprecated

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
To: linux-mips@linux-mips.org
Acked-by: Manuel Lauss <manuel.lauss@googlemail.com>
Patchwork: https://patchwork.linux-mips.org/patch/1427/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>