secure_buffer: Remove all-in-one buffer usage

Allocate objects via kzalloc() when possible. For the case where a large
chunk (512 Kb) of physically contiguous memory is required, continue to use
a special purpose buffer reserved at boot. Since this special purpose
buffer is no longer used for random metadata, the associated code is
simpler to understand.

Change-Id: Iaa39ea9497291238e2e5d38d52070364d8a5d31a
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>
Signed-off-by: Srinivasarao P <spathi@codeaurora.org>