of: reserved_mem: Check that memory reserved in device tree is in use

Inside of dma_alloc_attrs(), verify that devices with a reserved memory
region are actually using the region, in order to prevent wastage of
memory.

Change-Id: I56852a761d3253ec2a3a7db0f6501c73983e55c6
Signed-off-by: Chris Goldsworthy <cgoldswo@codeaurora.org>