davinci: add arch_ioremap() which uses existing static mappings

Add arch-specific ioremap() which uses any existing static mappings in
place of doing a new mapping.  From now on, drivers should always use
ioremap() instead of IO_ADDRESS().

In addition, remove the davinci_[read|write]* macros in favor of using
ioremap.

Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>