nvmem: layouts: sl28vpd: Use module_nvmem_layout_driver()

Stop open-coding the module init/exit functions. Use the
module_nvmem_layout_driver() instead.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>