genirq/msi: Add .msi_teardown() callback as the reverse of .msi_prepare()

While the MSI ops do have a .msi_prepare() callback that is responsible for
setting up the relevant (usually per-device) allocation, there is no
callback reversing this setup.

For this purpose, add .msi_teardown() callback.

In order to avoid breaking the ITS driver that suffers from related issues,
do not call the callback just yet.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20250513163144.2215824-2-maz@kernel.org