dts: silabs: interrupt levels corrected

Downgrading interrupt priority for non critical interrupts for the
supported SOCs.

Previously almost all interrupts were at level 0 i.e. meaning no interrupt
had priority over the others. In reality only radio interrupts are critical
while others can be served with less haste.

Now the level zero is reserved for interrupts that need higher priority
than the radio without bypassing irq locks (zero-latency interrupts)
Level 1 for radio and levels 2 and 3 for rest of the services.

Signed-off-by: Petri Pitkanen <petri.pitkanen@silabs.com>