dt-bindings: nvmem: imx-ocotp: update the binding to reflect data cells

imx-ocotp is implemented based on nvmem which can have data cells
as child node. Update the binding doc to reflect it to be more easily
understood by users.

Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>