remoteproc: add helper function to allocate rproc_mem_entry from reserved memory

remoteproc: add helper function to allocate rproc_mem_entry from reserved memory

This patch introduces rproc_res_mem_entry_init() helper function to
allocate a rproc_mem_entry structure from a reserved memory region.
In that case, rproc_mem_entry structure has no alloc and release ops.
It will be used to assigned the specified reserved memory to any
rproc sub device.
Relation between rproc_mem_entry and rproc sub device will be done
by name.

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