MAINTAINERS: Update BCM2711/BCM2835 maintainer

I haven't been able to find time to maintain BCM2711/BCM2835 these last
months, so it's only fair to pass the baton to Florian who's been doing
the work.

Signed-off-by: Nicolas Saenz Julienne <nsaenz@kernel.org>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>