remoteproc: fix rproc_da_to_va in case of unallocated carveout

remoteproc: fix rproc_da_to_va in case of unallocated carveout

With introduction of rproc_alloc_registered_carveouts() which
delays carveout allocation just before the start of the remote
processor, rproc_da_to_va() could be called before all carveouts
are allocated.
This patch adds a check in rproc_da_to_va() to return NULL if
carveout is not allocated.

Fixes: 284a4eeae6b5 ("remoteproc: add alloc ops in rproc_mem_entry struct")
Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>