lib: sbi: Add system_suspend_allowed domain property

Only privileged domains should be allowed to suspend the entire
system. Give the root domain this property by default and allow
other domains to be given the property by specifying it in the
DT.

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