dma-mapping-fast: Do not clobber existing atomic pool

The generic atomic pool is initialized at the postcore
initcall level. However, the same function is called to
initialize the atomic pool for fastmap at a later level,
and that function makes it so that the generic pool is
clobbered, and we start using the same atomic pool for
both fastmap and allocations outside of fastmap.

Fix this by adding support in the atomic pool framework
to create and utilize different atomic pools, and creating
a private atomic pool for fastmap.

Change-Id: I934774f44cc25ff66157be7920895618cf76b812
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>