Documentation: ABI: Add /sys/firmware/lefi/boardinfo description for Loongson64

Add a description for /sys/firmware/lefi/boardinfo on the Loongson
platform.
Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

Documentation/ABI/testing/sysfs-firmware-lefi-boardinfo

+What:		/sys/firmware/lefi/boardinfo
+Date:		October 2020
+Contact:	Tiezhu Yang <yangtiezhu@loongson.cn>
+Description:
+		Get mainboard and BIOS info easily on the Loongson platform,
+		this is useful to point out the current used mainboard type
+		and BIOS version when there exists problems related with
+		hardware or firmware.
+
+		The related structures are already defined in the interface
+		specification about firmware and kernel which are common
+		requirement and specific for Loongson64, so only add a new
+		boardinfo.c file in arch/mips/loongson64.
+
+		For example:
+
+		[loongson@linux ~]$ cat /sys/firmware/lefi/boardinfo
+		Board Info
+		Manufacturer            : LEMOTE
+		Board Name              : LEMOTE-LS3A4000-7A1000-1w-V01-pc
+		Family                  : LOONGSON3
+
+		BIOS Info
+		Vendor                  : Kunlun
+		Version                 : Kunlun-A1901-V4.1.3-20200414093938
+		ROM Size                : 4 KB
+		Release Date            : 2020-04-14
+
+		By the way, using dmidecode command can get the similar info if there
+		exists SMBIOS in firmware, but the fact is that there is no SMBIOS on
+		some machines, we can see nothing when execute dmidecode, like this:
+
+		[root@linux loongson]# dmidecode
+		# dmidecode 2.12
+		# No SMBIOS nor DMI entry point found, sorry.