mfd/sm501: depend on HAS_DMA

Currently the sm501 mfd driver can be compiled without any dependencies,
but through the use of dma_declare_coherent it really depends on
having DMA and iomem support.  Normally we don't explicitly require DMA
support as we have stubs for it if on UML, but in this case the driver
selects support for dma_declare_coherent and thus also requires
memmap support.  Guard this by an explicit dependency.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Lee Jones <lee.jones@linaro.org>