MAINTAINERS: replace non-matching patterns for loongson{2,3}

Commit ffe1f935

 ("MAINTAINERS: Add Loongson-2/Loongson-3 maintainers")
adds quite generic file entries for drivers/*/*loongson{2,3}* and
drivers/*/*/*loongson{2,3}* to be informed on changes to all loongson{2,3}
files in drivers.

However, only the pattern 'drivers/*/*loongson2*' matches to one file in
the repository, i.e., drivers/cpufreq/loongson2_cpufreq.c; all other
patterns have no file matches.

Hence, ./scripts/get_maintainer.pl --self-test=patterns complains:

  warning: no file matches    F:    drivers/*/*/*loongson2*
  warning: no file matches    F:    drivers/*/*/*loongson3*
  warning: no file matches    F:    drivers/*/*loongson3*

As in the last two and half years, no further files and drivers have
showed up to match those patterns, just name the one file that matches
explicitly and delete the others without a match.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>