ipc: Make the IPM instance index starting from 0

In the current code the naming of the
CONFIG_RPMSG_MULTI_INSTANCE_?_IPM_{TX,RX}_NAME symbol is 1-based. While
this is not currently an issue, it could easily become such if the
symbol is programmatically used as part of a preprocessor enumeration
(for example when using DT_INST_FOREACH_STATUS_OKAY(...) & co).

To avoid trouble, just make the index starting from 0 instead than 1.

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