riscv: use hart id instead of cpu id on machine_kexec

raw_smp_processor_id() doesn't return the hart id as stated in
arch/riscv/include/asm/smp.h, use smp_processor_id() instead
to get the cpu id, and cpuid_to_hartid_map() to pass the hart id
to the next kernel. This fixes kexec on HiFive Unleashed/Unmatched
where cpu ids and hart ids don't match (on qemu-virt they match).

Fixes: fba8a867

 ("RISC-V: Add kexec support")
Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>