MAINTAINERS: Update linux-fpga repository location

As maintainer team we have decided to move the linux-fpga development
to a shared repository with shared access.

Cc: Xu Yilun <yilun.xu@intel.com>
Cc: Wu Hao <hao.wu@intel.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>