drivers: power_domain: Introduce a gpio monitor driver

Power rails of some peripherals are controlled externally.
This is a case in embedded controllers, where the power of
some I2C devices are managed by the main application
processor.

To ensure that zephyr drivers access the devices where is
powered on, introduce a "monitoring" power domain. It works
by registering interrupt handler with gpio a pin, so that
when power state changes, it will notify relevant drivers.

Additionaly add CONFIG_POWER_DOMAIN_INIT_PRIORITY to replace
harcoded init priority.

Fixes: #51349

Signed-off-by: Albert Jakieła <jakiela@google.com>