mailbox: Use dev_fwnode()

mailbox: 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().

So use the dev_fwnode() helper.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Cc: Jassi Brar <jassisinghbrar@gmail.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>