misc: fastrpc: add mmap/unmap support

Support the allocation/deallocation of buffers mapped to the DSP.

When the memory mapped to the DSP at process creation is not enough,
the fastrpc library can extend it at runtime. This avoids having to do
large preallocations by default.

Signed-off-by: Jorge Ramirez-Ortiz <jorge.ramirez-ortiz@linaro.org>