FPGA Manager changes for 5.17-rc1

Russ' patches rework the way we register FPGA managers, regions and
bridges by simplifying the functions into a single register call.

Nathan's patch addresses an unused variable warning that was introduced
by Russ' patches.

Yang's patch addresses a kernel doc warning.

All patches have been reviewed on the mailing list, and have been in the
last few linux-next releases (as part of our for-next branch) without issues.

Signed-off-by: Moritz Fischer <mdf@kernel.org>
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