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>