nvmem: rockchip: add support for RK3368

This adds the necessary function for handling support on RK3368 SoCs

Signed-off-by: Romain Perier <romain.perier@collabora.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>