MAINTAINERS: Update fpga-mgr entry with new linux-fpga mailing list

Update the fpga-mgr framework entry with new linux-fpga@vger.kernel.org
mailing list.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
Cc: Alan Tull <atull@opensource.altera.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Michal Simek <monstr@monstr.eu>
Cc: Sören Brinkmann <soren.brinkmann@xilinx.com>
Cc: David S. Miller <davem@davemloft.net>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: linux-fpga@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>