irqchip/gic-v3: Rename GICv3 ITS MSI parent

The GICv5 ITS will reuse some GICv3 ITS MSI parent functions therefore
it makes sense to keep the code functionality in a compilation unit
shared by the two drivers.

Rename the GICv3 ITS MSI parent file and update the related
Kconfig/Makefile entries to pave the way for code sharing.

Signed-off-by: Lorenzo Pieralisi <lpieralisi@kernel.org>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250703-gicv5-host-v7-26-12e71f1b3528@kernel.org


Signed-off-by: Marc Zyngier <maz@kernel.org>