riscv: Fix linking error when building u-boot-spl with no SMP support

Switch off SMP support when building u-boot-spl would cause linking error as follow:
undefined reference to 'secondary hart relocate' and 'smp_call_function'.
Add macro to wrap up proper code region that needs SMP configuration on.

Signed-off by: Leo Liang <ycliang@andestech.com>
Cc: rick@andestech.com
Reviewed-by: Bin Meng <bin.meng@windriver.com>