Documentation/ABI: document the nvmem sysfs files

NVMEM has been in kernel since v4.2 but the ABI document was missing
since then. userspace interface did not change since then and seems
stable, so this patch documents the nvmem ABI.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>