RISC-V: smpboot: Create wrapper setup_smp()

setup_smp() currently assumes DT-based platforms. To enable ACPI,
first make this a wrapper function and move existing code to
a separate DT-specific function.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Palmer Dabbelt <palmer@rivosinc.com>