remoteproc: introduce rproc_add_carveout function

This patch introduces a new API to allow platform driver to register
platform specific carveout regions.

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>