MAINTAINERS: add an entry for the BCM2835 ARM sub-architecture

Add myself as the maintainer for the BCM2835 ARM support, and related
drivers. This is mainly so that the MAINTAINERS file contains some
relevant entry, and the rpi/ARM mailing lists; I'd be quite happy if
anyone else came along and wanted to maintain/co-maintain this.

Signed-off-by: Stephen Warren <swarren@wwwdotorg.org>