remoteproc: introduce rproc_find_carveout_by_name function

This patch provides a new function to find a carveout according
to a name.
If match found, this function returns a pointer on the corresponding
carveout (rproc_mem_entry structure).

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