bluetooth: hci: rpmsg: Use RPMsg Service

This patch modifies Bluetooth HCI RPMsg drivers and samples to use
RPMsg Service instead of configuring OpenAMP directly in the driver
or the sample.

Co-authored-by: Piotr Szkotak <piotr.szkotak@nordicsemi.no>
Signed-off-by: Hubert Miś <hubert.mis@nordicsemi.no>