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: I85078f351293b556a501e9aca075ef87e2e86afd
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>