kernel: Fix missing reference to team/thread in signal events.

The signal to the team/thread is only actually sent in a deferred
procedure. To ensure that the team/thread stays valid between the DPC
being queued and it actually running, we need to acquire a reference.

Fixes #11390, where the DPC was run after the team was already
destroyed.
Move ARM device tree files to an arch-specific subfolder

FDT are also used on PPC at least, and at least skeleton.dtsi
might clash since there is a different one for PPC.
norflash: Fix warning