riscv: Enable ARCH_SUSPEND_POSSIBLE for s2idle

riscv: Enable ARCH_SUSPEND_POSSIBLE for s2idle

With this configuration opened, the basic platform-independent s2idle is
provided by the sole "s2idle" string in `/sys/power/mem_sleep`.

At the end of s2idle, harts will hit the `wfi` instruction or enter the
SUSPENDED state through the sbi_cpuidle driver. The interrupt of possible
wakeup devices will be kept to wake the system up.

And platform-specific sleep states can be provided by future ACPI and
SBI SUSP extension support.

Signed-off-by: Song Shuai <songshuaishuai@tinylab.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20230529101524.322076-1-songshuaishuai@tinylab.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>
[hoegeun.kwon: cherry-pick the commit c1f048a6bd7d from mainline]
Signed-off-by: Hoegeun Kwon <hoegeun.kwon@samsung.com>
Change-Id: I535adff9152e6f20b9ad825b03cadfec6ac39684