soc: riscv: privileged: efinix-sapphire: Enable CONFIG_RISCV_HAS_CPU_IDLE

In an earlier commit, the riscv-privileged level implementation of
`arch_cpu_idle()` is not included unless `CONFIG_RISCV_HAS_CPU_IDLE` is
enabled.

This commit ensures the option is enabled on all the existing CPUs, thereby
maintaining the existing behavior.

Signed-off-by: Chen Xingyu <hi@xingrz.me>