struct powerdomain varialbes are all file local, make them static.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Reviewed-by: Jean Pihet <j-pihet@ti.com>
Reviewed-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Tested-by: Kevin Hilman <khilman@ti.com>
[khilman@ti.com: update changelog, drop error check in fast path]
Signed-off-by: Kevin Hilman <khilman@ti.com>