MAINTAINERS: add soc/ux500

get_maintainer.pl only returns linux-kernel@vger.kernel.org for the
drivers/soc/ux500/ files - fix it by adding drivers/soc/ux500/ entry

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Suggested-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>