dts: nrf54h20: Add zephyr,pm-device-runtime-auto; to uart instances

The uart driver for nRF54h20 doesn't call pm_device_runtime_enable().
During an uart driver init `pm_device_driver_init()` return early,
because the `pm_device_is_powered()` returns `false`. Power domains,
where uarts are instantiated, are disabled: `pm->domain->pm_base->state`
is not equal to `PM_DEVICE_STATE_ACTIVE`.

At the end of the day, an uart instance is left disabled.

This is a workaround to make the uart usable when CONFIG_PM,
CONFIG_PM_DEVICE and CONFIG_PM_DEVICE_RUNTIME are enabled.

Signed-off-by: Piotr Pryga <piotr.pryga@nordicsemi.no>