Bluetooth: controller: Introduction of RXFIFO composite

By use of the new macro RXFIFO_DEFINE, a complete set of data
structures for supporting a MEMQ'able FIFO with data pool backing is
created. This is intended for RX nodes such as done-events, RX PDU-
and ISO PDU events. The purpose is to simplify the data management
and code complexity, and provide re-usability.

This commit implements the existing done structures (MFIFO, mem pool,
mem link pool) as an RXFIFO. ISO is the next customer, and later the
pdu_rx_free MFIFO- and related pools may be refactored.

Signed-off-by: Morten Priess <mtpr@oticon.com>