maintainers: Step down as sub-maintainer

I'm giving full maintainership of the Raspberry Pi 3 platform port to
Paul. I'm also leaving the GXBB maintainership to Andre, who is also
happy to pass it on to someone else who is more interested in it.

Change-Id: Ieb2212f5fc11ebde9fc0c857e9e305d691d4ee3f
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>