Some architectures like PARISC is known not to support mmap properly
with the DMA buffer, where dma_mmap_coherent() returns -EINVAL
unconditionally.  From the API POV, we should rather drop the mmap
support there and expose it before the user-space tries to call mmap.

The patch contains again ugly ifdef's, unfortunately, as there is no
global flag indicating this.  Once when such macro is defined, we can
get rid of this instead.

Signed-off-by: Takashi Iwai <tiwai@suse.de>