memory: omap-gpmx: Use dev_fwnode()

irq_domain_create_simple() takes fwnode as the first argument. It can be
extracted from the struct device using dev_fwnode() helper instead of
using of_node with of_fwnode_handle().

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250611104348.192092-12-jirislaby@kernel.org


[krzk: Dropped redundant parts of commit msg]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>