arm/dt: Convert QSECOM heap to CMA

The QSECOM heap is currently carved out of system memory.
The heap is used infrequently and there isn't a time
constraint on allocations so there isn't a need for it
to be completely removed. Convert the heap to be backed
by CMA underneath to save memory.

Change-Id: I4ec8adb99b5a8c5a95990ce715a803e1adf71ad0
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>