Documentation: fpga: cleanup

Minor fixes including:

* fix some typos
* correct use of a/an
* rephrase explanation of .state ops function
* s/re-use/reuse/ (use only one spelling of 'reuse' in these docs)
* s/cpu/CPU/

Signed-off-by: Alan Tull <atull@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>