dt-bindings: fsl: scu: add ocotp binding

NXP i.MX8QXP is an ARMv8 SoC with a Cortex-M4 core inside as
system controller(SCU), the ocotp controller is being controlled
by the SCU, so Linux need use RPC to SCU for ocotp handling. This
patch adds binding doc for i.MX8 SCU OCOTP driver.

Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Ulf Hansson <ulf.hansson@linaro.org>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Anson Huang <anson.huang@nxp.com>
Cc: devicetree@vger.kernel.org
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>