remoteproc: st: add reserved memory support

ST remote processor needs some specified memory regions for
firmware and IPC.
Memory regions are defined as reserved memory and should
be registered in remoteproc core thanks to rproc_add_carveout
function before rproc_start. For this, st rproc driver implements
prepare ops.

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