Add a private iommu pointer to the ARM-specific arch data in the
device struct, which will be used to attach iommu-specific data
to devices which require iommu support.

Different iommu implementations (on different platforms) will attach
different types of data to this pointer, so 'void *' is currently used
(the downside is reduced typesafety).

Note: ia64, x86 and sparc have this exact iommu extension as well, and
if others are likely to adopt it too, we might want to consider
adding this to the device struct itself directly.

Signed-off-by: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>