riscv: Add stimecmp save and restore

If the HW support the SSTC extension, we should save and restore the
stimecmp register while cpu non retention suspend.

Signed-off-by: Nick Hu <nick.hu@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Link: https://lore.kernel.org/r/20250219114135.27764-2-nick.hu@sifive.com


Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>