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>

drivers/mfd/Kconfig

@@ -1066,6 +1066,7 @@ config MFD_SI476X_CORE
 
 config MFD_SM501
 	tristate "Silicon Motion SM501"
+	depends on HAS_DMA
 	 ---help---
 	  This is the core driver for the Silicon Motion SM501 multimedia
 	  companion chip. This device is a multifunction device which may