ARM: tegra: annotate IO_*_VIRT pointers

Provide __iomem annotation for IO_*_VIRT pointers, which will propagate
up through IO_TO_VIRT(). Also fixes a 0 to NULL conversion of the base
case to silence sparse.

Unfortunately map_desc takes an unsigned long for the pointer instead of
a void __iomem *. For now, cast explicitly for those cases.

v2: change define to use IOMEM() like many other mach platforms per
comment from Russell.

Signed-off-by: Olof Johansson <olof@lixom.net>
Acked-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>