Documentation: ABI: nvmem: add documentation for JZ4780 efuse ABI

This patch brings support for the JZ4780 efuse. Currently it only exposes
a read only access to the entire 8K bits efuse memory.
Tested-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
Signed-off-by: Mathieu Malaterre <malat@debian.org>
Reviewed-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Link: https://lore.kernel.org/r/20200310132257.23358-14-srinivas.kandagatla@linaro.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/ABI/testing/sysfs-driver-jz4780-efuse

+What:		/sys/devices/*/<our-device>/nvmem
+Date:		December 2017
+Contact:	PrasannaKumar Muralidharan <prasannatsmkumar@gmail.com>
+Description:	read-only access to the efuse on the Ingenic JZ4780 SoC
+		The SoC has a one time programmable 8K efuse that is
+		split into segments. The driver supports read only.
+		The segments are
+		0x000   64 bit Random Number
+		0x008  128 bit Ingenic Chip ID
+		0x018  128 bit Customer ID
+		0x028 3520 bit Reserved
+		0x1E0    8 bit Protect Segment
+		0x1E1 2296 bit HDMI Key
+		0x300 2048 bit Security boot key
+Users:		any user space application which wants to read the Chip
+		and Customer ID