MAINTAINERS: Remove Grant Likely

Unfortunately, I'm no longer to spend the time needed on maintainership.
It is time for me to step aside and pass maintainership to other
engineers. I'm not disappearing from Linux development, but it would be
irresponsible for me to hold onto a job that I am unable to do.

v2: Leave my name on devicetree core code maintainership. Rob NAKed that
    part of the patch. :)

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Cc: Linux Walleij <linus.walleij@linaro.org>
Cc: Mark Brown <broonie@kernel.org>
Cc: Rob Herring <rob.herring@calxeda.com>