caif: include linux/io.h

The caif_shmcore requires io.h in order to use ioremap, so include that
explicitly to compile in all configurations.

Also add a note about the use of ioremap(), which is not a proper way
to map a DMA buffer into kernel space. It's not completely clear what
the intention is for using ioremap, but it is clear that the result
of ioremap must not simply be accessed using kernel pointers but
should use readl/writel or memcopy_{to,from}io. Assigning the result
of ioremap to a regular pointer that can also be set to something
else is not ok.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>