MAINTAINERS: Add catch all for boards to the arch maintainers

Add a catch all for all architectures for their relevant board
directories so reviews, etc get covered/seen by the arch maintainers as
well.

Change-Id: Ib5a82039d7e8136bb73722df0a5edde0329726e8
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>