drivers: gpio: silabs: Support EM4 wakeup interrupts

EM4 wakeup interrupts are dedicated interrupts tied to specific
pins that enable wakeup from EM4 through reset. Add support
for using these interrupts instead of the regular interrupts when
the GPIO_INT_TRIG_WAKE flag is set.

Since it's not possible to tell what pin is associated with what
EM4WU interrupt at runtime, the driver must store a mapping table
sourced from device tree.

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