hw/block/pflash_cfi02: Document 'Page Mode' operations are not supported

hw/block/pflash_cfi02: Document 'Page Mode' operations are not supported

The 'page mode' feature entry was implicitly set as zero
(not supported). Document it exists, so we won't discard
it if we squeeze the CFI table.

Signed-off-by: Stephen Checkoway <stephen.checkoway@oberlin.edu>
Message-Id: <20190426162624.55977-6-stephen.checkoway@oberlin.edu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: Extracted from bigger patch]
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>