ipc: rpmsg_multi_instance: Use only rpsmg_mi_ctx_cfg for configuration

For the instance configuration the rpmsg_multi_instance code is
currently using a set of configuration info coming from two different
sources: the rpsmg_mi_ctx_cfg struct and Kconfig.

This is not only confusing but it's preventing to configure the
instances using information not coming from Kconfig (for example if we
want to configure the instance using DT).

Signed-off-by: Carlo Caione <ccaione@baylibre.com>