remoteproc: add release ops in rproc_mem_entry struct

Memory entry could be allocated in different ways (ioremap,
dma_alloc_coherent, internal RAM allocator...).
This patch introduces a release ops in rproc_mem_entry structure
to associate dedicated release mechanism to each memory entry descriptor
in order to keep remoteproc core generic.

Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Acked-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>