MIPS: Loongson64: Add /sys/firmware/lefi/boardinfo

Add /sys/firmware/lefi/boardinfo to 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.

E.g. with this patch:

[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.

Signed-off-by: Tiezhu Yang <yangtiezhu@loongson.cn>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>