Add IO mapping of SRAM for CPU access

To allow the CPU to directly access the NPU's SRAM, the SRAM has been IO
mapped. The size of the mapping and the SRAM has been limited to the
maximum size the NPU can handle to not make more memory available than
can be used.

Change-Id: Ifc7bfdb8934a67ae53d30243ea4ee05fefbfd18a
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>