nvmem: Document type attribute

nvmem: Document type attribute

Document a type attribute used by userspace to discern different types of
NVMEM devices. The implementation is already present, the ABI document is
missing, add it.

Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20240705074852.423202-14-srinivas.kandagatla@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>