fpga: dfl: afu: add STP (SignalTap) support

STP (SignalTap) is one of the private features under the port for
debugging. This patch adds private feature driver support for it
to allow userspace applications to mmap related mmio region and
provide STP service.

Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Wu Hao <hao.wu@intel.com>
Acked-by: Moritz Fischer <mdf@kernel.org>
Acked-by: Alan Tull <atull@kernel.org>
Signed-off-by: Moritz Fischer <mdf@kernel.org>