fpga: Add support for Xilinx DFX AXI Shutdown manager

This patch adds support for Xilinx Dynamic Function eXchange(DFX) AXI
shutdown manager IP. It can be used to safely handling the AXI traffic
on a Reconfigurable Partition when it is undergoing dynamic reconfiguration
and there by preventing system deadlock that may occur if AXI transactions
are interrupted during reconfiguration.

PR-Decoupler and AXI shutdown manager are completely different IPs.
But both the IP registers are compatible and also both belong to the
same sub-system (fpga-bridge).So using same driver for both IP's.

Signed-off-by: Nava kishore Manne <nava.manne@xilinx.com>
Reviewed-by: Tom Rix <trix@redhat.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>