cpuidle: riscv: support non-SMP config

Add <asm/smp.h> for cpuid_to_hartid_map etc.
This is needed for both SMP and non-SMP builds, but not having it
causes a build error for non-SMP:

drivers/cpuidle/cpuidle-riscv-sbi.c: In function 'sbi_cpuidle_init_cpu':
drivers/cpuidle/cpuidle-riscv-sbi.c:350:26: error: implicit declaration of function 'cpuid_to_hartid_map' [-Werror=implicit-function-declaration]

Fixes: 6abf32f1 ("cpuidle: Add RISC-V SBI CPU idle driver")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>