ARM: OMAP4+: PM: Use only valid low power state for CPU hotplug

Not all SoCs support OFF mode - for example DRA74/72. So, use valid
power state during CPU hotplug.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Kevin Hilman <khilman@linaro.org>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>