irqdomain: Constify parameter in is_fwnode_irqchip()

The fwnode parameter has no reason to be a pointer to an un-const struct
fwnode_handle. Indeed, struct fwnode_handle is not supposed to be modified
by the function.

Be consistent with other function performing the same kind of operation
such as is_of_node(), is_acpi_device_node() or is_software_node(): constify
the fwnode parameter.

Signed-off-by: Herve Codina <herve.codina@bootlin.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240614173232.1184015-5-herve.codina@bootlin.com