Used to support runtime pm.

Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Michael Turquette <mturquette@baylibre.com>
Reviewed-by: NStephen Boyd <sboyd@kernel.org>
Signed-off-by: NDong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: NShawn Guo <shawnguo@kernel.org>