hw/block/pflash_cfi02: Implement multi-sector erase

hw/block/pflash_cfi02: Implement multi-sector erase

After two unlock cycles and a sector erase command, the AMD flash chips
start a 50 us erase time out. Any additional sector erase commands add a
sector to be erased and restart the 50 us timeout. During the timeout,
status bit DQ3 is cleared. After the time out, DQ3 is asserted during
erasure.

Signed-off-by: Stephen Checkoway <stephen.checkoway@oberlin.edu>
Message-Id: <20190426162624.55977-9-stephen.checkoway@oberlin.edu>
Acked-by: Thomas Huth <thuth@redhat.com>
Acked-by: Philippe Mathieu-Daudé <philmd@redhat.com>
[PMD: Rebased]
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>