dts: arm: silabs: Add EM4 wakeup pins and power state

Add EM4 wakeup capable pin mapping to GPIO port node. Pins
capable of EM4 wakeup have dedicated interrupt flags.

Add EM4 as a soft-off power state that is disabled by default.
Marking it as disabled allows users to enter it with
`pm_state_force()`, while preventing the power management
subsystem from selecting the state automatically.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>