arch/riscv: Boot secondary CPUs for SMP support

Secondary CPUs are now initialised and made available to the system. If
the system has more CPUs than configured via CONFIG_MP_NUM_CPUS, those
are still left looping as before.

Some implementations of `soc_interrupt_init` also changed to use
`arch_irq_lock` instead of `irq_lock`.

Signed-off-by: Ederson de Souza <ederson.desouza@intel.com>