Bluetooth: Add priority buffer for handling critical HCI events

Command Status, Command Complete and Number of Complete Packets are
used for driving TX fibers (HCI and connection). Missing any of
those event will lead to HCI traffic stall.

Priority pool will be used when critical HCI event is received
or normal pool runs out of buffers. The difference with priority
pool is that buffer from it must not be passed to RX fiber and
must be freed from bt_recv.

If driver knows HCI event opcode before requesting buffer for event
it may hint HCI core about preferred pool selection.

Change-Id: Iad14724945bb59721c5ffb6b62d5a8a3e3f70be7
Signed-off-by: Szymon Janc <ext.szymon.janc@tieto.com>