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>

diff --git a/Documentation/ABI/testing/sysfs-firmware-lefi-boardinfo b/Documentation/ABI/testing/sysfs-firmware-lefi-boardinfo
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+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.