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>

Author: Marek Vasut

Documentation/ABI/stable/sysfs-bus-nvmem

@@ -20,3 +20,14 @@ Description:
 		  ...
 		  *
 		  0001000
+
+What:		/sys/bus/nvmem/devices/.../type
+Date:		November 2018
+KernelVersion:	5.0
+Contact:	Alexandre Belloni <alexandre.belloni@bootlin.com>
+Description:
+		This read-only attribute allows user to read the NVMEM
+		device type. Supported types are "Unknown", "EEPROM",
+		"OTP", "Battery backed", "FRAM".
+		Note: This file is only present if CONFIG_NVMEM_SYSFS
+		is enabled.