lib: sbi: Update system suspend to spec

commit 68e66106120f ("SUSP: Add SBI_ERR_DENIED") of the SBI spec adds
a new error code, SBI_ERR_DENIED, which is returned when entry criteria
has not be meant. Update the system suspend implementation to return
this error when it has detected that not all harts are in the STOPPED
state.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Reviewed-by: Anup Patel <anup@brainfault.org>