embassy-rp: allow injection of FIFO handler

[kaspar030]

Add fifo-handler feature for injecting a custom handler of FIFO messages.
Also enable FIFO queue in core1->core0 direction.

In Ariel OS, we need a way to trigger the scheduler on the other core. The FIFO peripheral is made for this, but also claimed by embassy-rp, for stopping the second core from the first. Inside embassy, I think this is used to stop execution from flash while messing with it.

This PR (optionally, based on fifo-handler feature) makes this a bit more flexible:

1. It is now also possible to stop the first core from the second
2. With fifo-handler enabled, an fn handle_fifo(u32) -> bool is called for each FIFO message arriving from the other core that is not used for stopping or resuming.

Signed-off-by: Kaspar Schleiser [email protected]

[CBJamo]

I like this change.

Separately, on the 2350, pausing/resuming the cores should probably be switched to use a pair of doorbells rather than the fifo. Possibly that would work for your scheduler case as well. Then the fifos could be left free for users.