riscv: Deduplicate code in setup_smp()

Both the ACPI and DT implementations contain some of the same code.
Move it to the calling function so it is not duplicated.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20231121234736.3489608-2-samuel.holland@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>