fwk: Attach `malloc` attribute to allocators

To aid in optimization efforts done by the compiler, we attach the
`malloc` attribute to any memory allocation functions defined by us to
indicate that the pointer returned by the function does not alias with
any other pointer.

Change-Id: Ie66cac0b7e1bb4cf5e70f523c6917da99942fbb2
Signed-off-by: Chris Kay <chris.kay@arm.com>