ion: cma_secure_heap: validate buffer length

The secure CMA heap only supports buffers lengths that are multiples of
1MB. Currently, we simply crash if a bad length is requested. Rather
than crashing, log an error and return a failure.

Change-Id: I0387ee9f2900e44c817d3a7497e9b0528dd8c33b
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>