dma: select GENERIC_ALLOCATOR for DMA_REMAP

When DMA_REMAP is enabled, code in remap.c needs generic allocator.
It currently worked since few architectures uses it (arm64, csky) and
they both select GENERIC_ALLOCATOR. Select it when using DMA_REMAP
to have correct dependencies.

Signed-off-by: Clement Leger <clement.leger@kalray.eu>
Signed-off-by: Christoph Hellwig <hch@lst.de>