lib: sbi_hsm: handle failure when hart_stop returns SBI_ENOTSUPP

Make use of generic warm-boot path when platform hart_stop callback
returns SBI_ENOTSUPP, in case certain hart can not turn off its
power domain, or it detects some error occured in power management
unit, it can fall through warm-boot flow and wait for interrupt in
sbi_hsm_hart_wait().

Also improves comment in sbi_hsm_hart_wait().

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Atish Patra <atishp@rivosinc.com>
Reviewed-by: Anup Patel <anup@brainfault.org>