MPFS implements external interrupt control on a per-hart basis i.e. there
are PLIC control registers for each hart separately. This means we need
a procedure to initialize such registers for each hart individually,
instead of only for the boot hart like it is now.
Fix this by implementing mpfs_plic_init_hart which can be called by each
hart as needed.
Note: it is not a good idea to initialize all harts from the boot hart,
as the boot hart may not know which harts are used by NuttX in AMP
configuration. It is better that the hart initializes itself.
Note: The hartid must be provided as explicit parameter, as it cannot
be queried via riscv_mhartid() yet; the per-cpu structure is initialized
later on which means riscv_mhartid() would return 0 for all harts except
the boot hart.
cf95305934