feat: enable polling support for k_pipe interface

This commit adds polling support to the newly rewritten k_pipe interface.
Changes include:

* Removed ifdef CONFIG_POLL from kernel/poll.c to let both implementations
  coexist.
* Added the needed datastructures to the new k_pipe struct.
* k_pipe_write(..) now notifies the poll subsystem that data is available.

Signed-off-by: Måns Ansgariusson <Mansgariusson@gmail.com>