ipc: Call ipm_set_enabled() on RX channel also for dual IPM support

The IPC drivers rpmsg_service and rpmsg_multi_instance are not
explicitly enabling the RX IPM channel when two different devices are
used for TX and RX. While this could be redundant for some IPM drivers,
in some cases the hardware needs to be enabled before using it.

Add the missing calls to ipm_set_enabled() for both the devices.

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