Bluetooth: Controller: replace work with mayfly

This commit introduces a very light-weight and lock-less
scheduling of the Controller's deferred function calls
called the Mayfly.

Earlier work implementation used in the Controller had an
O(n) to schedule a function in a linked list that used IRQ
lock during modification of the linked list in the
Controller's ISR executions.

Mayfly is a compile time configurable matrix of queues
where an execution context-safe queue exists between two
execution context, one being the caller and the second
being the callee. Callee(s) are run in a software interrupt
but can also be run in an OS thread.

There are minor clean ups too in this commit related to
folder structure.

Change-id: I5ff44dcee6679d2f5ce9e8437d98d6c868782f3d
Signed-off-by: Vinayak Chettimada <vinayak.kariappa.chettimada@nordicsemi.no>