Move Documentation/fpga/fpga-region.txt to
driver-api/fpga/fpga-region.rst. Including:
- Add it to driver-api/fpga/index.rst
- Formatting changes to build cleanly as ReST documentation
- Some rewrites for better flow as a ReST doc such as moving
API reference to the end of the doc
- Rewrite API reference section to refer to kernel-doc
documentation in fpga-region.c driver code

Signed-off-by: Alan Tull
Signed-off-by: Greg Kroah-Hartman