ARM: dts: imx6ul: use nvmem-cells for cpu speed grading

commit 92f0eb08 upstream.

On i.MX6UL, accessing OCOTP directly is wrong because the ocotp clock
needs to be enabled first, so use the nvmem-cells binding instead.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>
Cc: Sébastien Szymanski <sebastien.szymanski@armadeus.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>