samples: openamp_rsc_table: Add the option to use predefined vring ID

Currently, Zephyr is always sending back notifications to
AP (e.g Linux in our case) on channel 0.

But this currently doesn't work if Linux uses other channel
id for communication. So, add option to use predefined
vring ID that can accomodate Linux used ID.

Signed-off-by: Alexandru Lastur <alexandru.lastur@nxp.com>