armv7R: K3: am654: Use full malloc implementation in SPL

Switch to using the full malloc scheme in post-relocation SPL to allow
better utilization of available memory for example by allowing memory
to get freed. Initially allocate a 16MB-sized region in DDR starting
at address 0x84000000 for this purpose.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>