irqchip/mips-gic: Prevent indirect access to clusters without CPU cores

It is possible to have zero CPU cores in a cluster; in such cases, it is
not possible to access the GIC, and any indirect access leads to an
exception.

Prevent access to such clusters by checking the number of cores in the
cluster at all places which issue indirect cluster access.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Signed-off-by: Aleksandar Rikalo <arikalo@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/20241028175935.51250-14-arikalo@gmail.com