clk: imx: gate2: Add locking in is_enabled op

Protect against enabling/disabling the gate while we're
checking if it is enabled.

Signed-off-by: Abel Vesa <abel.vesa@nxp.com>
Reviewed-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>