fpga: dfl: move dfl bus related APIs to include/linux/dfl.h

fpga: dfl: move dfl bus related APIs to include/linux/dfl.h

Now the dfl drivers could be made as independent modules and put in
different folders according to their functionalities. In order for
scattered dfl device drivers to include dfl bus APIs, move the
dfl bus APIs to a new header file in the public folder.

[mdf@kernel.org: Fixed up header guards to match filename]

Reviewed-by: Tom Rix <trix@redhat.com>
Acked-by: Wu Hao <hao.wu@intel.com>
Signed-off-by: Xu Yilun <yilun.xu@intel.com>
Signed-off-by: Moritz Fischer <mdf@kernel.org>
Link: https://lore.kernel.org/r/20210107043714.991646-7-mdf@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>