RISC-V: Rename and move plic_find_hart_id() to arch directory

The plic_find_hart_id() can be useful to other interrupt controller
drivers (such as RISC-V local interrupt driver) so we rename this
function to riscv_of_parent_hartid() and place it in arch directory
along with riscv_of_processor_hartid().

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Acked-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>