vcsm: VideoCore shared memory service for BCM2835

Add experimental support for the VideoCore shared memory service.
This allows user processes to allocate memory from VideoCore's
GPU relocatable heap and mmap the buffers. Additionally, the memory
handles can passed to other VideoCore services such as MMAL, OpenMax
and DispmanX

TODO
* This driver was originally released for BCM28155 which has a different
  cache architecture to BCM2835. Consequently, in this release only
  uncached mappings are supported. However, there's no fundamental
  reason which cached mappings cannot be support or BCM2835
* More refactoring is required to remove the typedefs.
* Re-enable the some of the commented out debug-fs statistics which were
  disabled when migrating code from proc-fs.
* There's a lot of code to support sharing of VCSM in order to support
  Android. This could probably done more cleanly or perhaps just
  removed.

Signed-off-by: Tim Gover <timgover@gmail.com>

config: Disable VC_SM for now to fix hang with cutdown kernel