fpga: dfl: afu: expose __afu_port_enable/disable function.

As these two functions are used by other private features within the
same driver module but different driver files. e.g. in error reporting
private feature, it requires to clear errors when port is in reset.

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>