fpga: dfl: afu: support debug access to memory-mapped afu regions

Allow debug access to memory-mapped regions using e.g. gdb.

Signed-off-by: Dominic Chen <d.c.ddcc@gmail.com>
Acked-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>