README.evbmips for kernel configs

Hi,

I am preparing README file for evbmips kernel configs (in similar
fashion as current evbarm counterpart
https://nxr.netbsd.org/xref/src/sys/arch/evbarm/conf/README.evbarm)
and need some help and wrapping in up. You can review the current
draft committed here:

https://github.com/vezhlys/netbsd-src/blob/evbmipsreadme/sys/arch/evbmips/conf/README.evbmips

The main discussion went around MALTA32/XLSATX32 which are actually
64-bit kernel currently despite their 32 suffix. I had an assumption
that they target 32-bit userland, however it may be incorrect
assumption. Maybe somebody knows what is their actual purpose? Any
additional comment are welcome on how to improve this README before I
commit a final version. Thank you.

Regards,
Andrius V