MAINTAINERS: Add self as extended maintainer for a slew of files

Take over sole maintenance of Nomadik, U300 and Ux500. Since all are
Device Tree converted and using standard format drivers this is not
burdensome. Alessandro is not working on this platform any more.
Let's use one single git tree for all of them and combine the
MAINTAINERS entries into one.

Suggested-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Alessandro Rubini <rubini@unipv.it>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>