soc: ti: ti_sci_inta_msi: Rework MSI descriptor allocation

Protect the allocation properly and use the core allocation and free
mechanism.

No functional change intended.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20211206210748.737904583@linutronix.de