mailbox: mchp-ipc-sbi: fix out-of-bounds access in mchp_ipc_get_cluster_aggr_irq()

In the Linux kernel, the following vulnerability has been resolved:

mailbox: mchp-ipc-sbi: fix out-of-bounds access in mchp_ipc_get_cluster_aggr_irq()

The cluster_cfg array is dynamically allocated to hold per-CPU
configuration structures, with its size based on the number of online
CPUs. Previously, this array was indexed using hartid, which may be
non-contiguous or exceed the bounds of the array, leading to
out-of-bounds access.
Switch to using cpuid as the index, as it is guaranteed to be within
the valid range provided by for_each_online_cpu().