nvmem: Add macro to register nvmem layout drivers

Provide a module_nvmem_layout_driver() macro at the end of the
nvmem-provider.h header to reduce the boilerplate when registering nvmem
layout drivers.

Suggested-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>