[MTD] Introduce MTD_BIT_WRITEABLE

o Add a flag MTD_BIT_WRITEABLE for devices that allow single bits to be
  cleared.
o Replace MTD_PROGRAM_REGIONS with a cleared MTD_BIT_WRITEABLE flag for
  STMicro and Intel Sibley flashes with internal ECC.  Those flashes
  disallow clearing of single bits, unlike regular NOR flashes, so the
  new flag models their behaviour better.
o Remove MTD_ECC.  After the STMicro/Sibley merge, this flag is only set
  and never checked.

Signed-off-by: Joern Engel <joern@wh.fh-wedel.de>