sparse: add an address space and a __sparse_force annotation

We want to use a sparse address space to identify invalid conversions
between cached and uncached address aliases. This patch adds a
__sparse_cache sparse annotation for that. Where those conversions
must be done that has to be supported by using the __sparse_force
sparse attribute. To avoid compiler complains about unknown
attributes we add a -Wno-attributes flag when building with sparse
support.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>