msm: SSR: Don't fail init on NULL restart orders

The subsystem framework works fine without a restart_order for a
particular machine. Therefore, always return success if there are
no restart orders. This allows 8974 to start using the subsystem
restart framework while we work on removing the need for the
restart order list.

Change-Id: Ia644f7420752c12dcb1cf606591a59b1c3caa8e6
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>