To support per device MSI domains the ITS must provide MSI parent domain
functionality.

Provide the basic skeleton for this:

   - msi_parent_ops
   - child domain init callback
   - the MSI parent flag set in irqdomain::flags

This does not make ITS a functional parent domain as there is no bit set in
the bus_select_mask yet, but it provides the base to implement PCI and
platform MSI support gradually on top.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Anna-Maria Behnsen <anna-maria@linutronix.de>
Signed-off-by: Shivamurthy Shastri <shivamurthy.shastri@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20240623142234.903076277@linutronix.de