RISC-V: cpuidle: fix Kconfig select for RISCV_SBI_CPUIDLE

There can be lots of build errors when building cpuidle-riscv-sbi.o.
They are all caused by a kconfig problem with this warning:

WARNING: unmet direct dependencies detected for RISCV_SBI_CPUIDLE
  Depends on [n]: CPU_IDLE [=y] && RISCV [=y] && RISCV_SBI [=n]
  Selected by [y]:
  - SOC_VIRT [=y] && CPU_IDLE [=y]

so make the 'select' of RISCV_SBI_CPUIDLE also depend on RISCV_SBI.

Fixes: c5179ef1

 ("RISC-V: Enable RISC-V SBI CPU Idle driver for QEMU virt machine")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: kernel test robot <lkp@intel.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>