ARM: dts: imx6sl: add clocks property for ocotp node

Add clocks property for ocotp node.

Signed-off-by: Peng Fan <van.freenix@gmail.com>
Cc: Sascha Hauer <kernel@pengutronix.de>
Cc: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>