dma-mapping: improve selection of dma_declare_coherent availability

This API is primarily used through DT entries, but two architectures
and two drivers call it directly.  So instead of selecting the config
symbol for random architectures pull it in implicitly for the actual
users.  Also rename the Kconfig option to describe the feature better.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com> # MIPS
Acked-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/linux/device.h

@@ -1017,7 +1017,7 @@ struct device {
 
 	struct list_head	dma_pools;	/* dma pools (if dma'ble) */
 
-#ifdef CONFIG_HAVE_GENERIC_DMA_COHERENT
+#ifdef CONFIG_DMA_DECLARE_COHERENT
 	struct dma_coherent_mem	*dma_mem; /* internal for coherent mem
 					     override */
 #endif