Split DMA memory and buffer setup in kernel driver

To allow the NPU kernel driver to allocate and use DMA memory internally
without creating a buffer instance, the DMA memory management has been
split out from the buffer code.

Change-Id: I46fdeee51b5ef786a54b8e7c866d137d91222724
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>