remoteproc: add helper function to allocate and init rproc_mem_entry struct

This patch introduces rproc_mem_entry_init helper function to
simplify rproc_mem_entry structure allocation and filling by
client.

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