[media] st-delta: add memory allocator helper functions

Helper functions used by decoder back-ends to allocate
physically contiguous memory required by hardware video
decoder.

Acked-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Hugues Fruchet <hugues.fruchet@st.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>