MAINTAINERS: fpga: update email and directory paths

A few updates:
* Updating my email address
* Adding another docs directory: Documentation/fpga
* Making the include path not specific to fpga-mgr.h only

Signed-off-by: Alan Tull <atull@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>