dt-bindings: nvmem: add binding for Rockchip OTP controller

Newer Rockchip SoCs use a different IP for accessing special one-
time-programmable memory, so add a binding for these controllers.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>