nvmem: rockchip-efuse: describe the usage of eFuse

This patch add the bindings document of rockchip eFuse driver.

Cc: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Cc: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: ZhengShunQian <zhengsq@rock-chips.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>