boards: nordic: add mx25uw6345g flash node to nRF54H20dk

The nRF54H20dk has mx25uw6345g flash device on board. Added DTS
description and pinctrl configuration.

The flash device node is disabled by default. The flash device needs to
be powered-on using the nRF Connect for Desktop Board Configurator
application.

Signed-off-by: Rafał Kuźnia <rafal.kuznia@nordicsemi.no>