MAINTAINERS: fpga: hand off maintainership to Moritz

I'm moving on to a new position and stepping down as FPGA subsystem
maintainer.  Moritz has graciously agreed to take over the
maintainership.

Signed-off-by: Alan Tull <atull@kernel.org>
Acked-by: Moritz Fischer <mdf@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>