mm/memblock: add memblock_get_current_limit

Apart from setting the limit of memblock, it's also useful to be able
to get the limit to avoid recalculating it every time. Add the function
to do so.

Change-Id: I4f28dc1e549fd4c7fabf4e0dbd97871dbaa318ab
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>