remoteproc: Allow overriding only sanity_check

Now that rproc_elf_sanity_check can be used by external drivers, allow
to only overwrite the sanity_check member of rproc_ops. This will allow
drivers to handle elf32 and elf64 by overwriting sanity_check with
rproc_elf_sanity_check function.

Signed-off-by: Clement Leger <cleger@kalray.eu>
Link: https://lore.kernel.org/r/20200302093902.27849-8-cleger@kalray.eu


Signed-off-by: Bjorn Andersson <bjorn.andersson@linaro.org>