nvmem: sunxi-sid: convert to SPDX license tags

Updates license to use SPDX-License-Identifier.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>