irqchip/gic-v3-its: Switch platform MSI to MSI parent

Similar to the previous conversion of the PCI/MSI support lift the
prepare() callback from the existing platform MSI code and enable
platform MSI and the related device domain bus tokens in select
and the child domain initialization code.

All platform MSI users are automatically using the new per device MSI model
now.

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/20240623142235.271734124@linutronix.de