MAINTAINERS: Add Alexandre Belloni as an RTC maintainer

I've noticed that most of the patches for the RTC subsystem are currently
either taken directly by Andrew or going through another maintainer's
tree, quite often without an Acked-by or Reviewed-by tag.

I'd like to propose myself as the RTC subsystem co-maintainer, to mainly
help Alessandro reviewing incoming patches and maintain a subsystem tree
to avoid having the RTC patches going through trees when they have no
particular dependencies.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>