fmem: Add reserved_size to fmem

Due to a limitation in how the video hardware is able
to access memory, fmem needs to be able to allocate a
physically contiguous block of memory that can be split into
two pieces. One piece will be managed by fmem
while the other piece (reserved) will not be handled by fmem.
The hardware limitation requires the reserved part to be at
a lower address than the other piece managed by fmem.

Add a new parameter to fmem that allows platform
data to specify the size of the piece that should not
be managed by fmem.

Change-Id: I910680ca72233de9cef91ac3262a078a36f1b1fd
Signed-off-by: Olav Haugan <ohaugan@codeaurora.org>