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>

Documentation/ABI/stable/sysfs-bus-nvmem

+What:		/sys/bus/nvmem/devices/.../nvmem
+Date:		July 2015
+KernelVersion:  4.2
+Contact:	Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
+Description:
+		This file allows user to read/write the raw NVMEM contents.
+		Permissions for write to this file depends on the nvmem
+		provider configuration.
+
+		ex:
+		hexdump /sys/bus/nvmem/devices/qfprom0/nvmem
+
+		0000000 0000 0000 0000 0000 0000 0000 0000 0000
+		*
+		00000a0 db10 2240 0000 e000 0c00 0c00 0000 0c00
+		0000000 0000 0000 0000 0000 0000 0000 0000 0000
+		...
+		*
+		0001000

MAINTAINERS

@@ -9466,6 +9466,7 @@ M:	Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
 S:	Maintained
 F:	drivers/nvmem/
 F:	Documentation/devicetree/bindings/nvmem/
+F:	Documentation/ABI/stable/sysfs-bus-nvmem
 F:	include/linux/nvmem-consumer.h
 F:	include/linux/nvmem-provider.h