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.orgSigned-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Kconfig

@@ -873,6 +873,9 @@ config ARCH_HIBERNATION_POSSIBLE
 config ARCH_HIBERNATION_HEADER
 	def_bool HIBERNATION
 
+config ARCH_SUSPEND_POSSIBLE
+	def_bool y
+
 endmenu # "Power management options"
 
 menu "CPU Power Management"