Flash handling for STM32F76x/77x and F446 added

Flash handling for STM32F76x/77x and F446 added

- added ids for various parts
- rewrite of sector allocation to deal with dual-bank F76x/77x
- single- / dual-bank mode for F76x/77x
- sector protection adapted for F76x/77x in dual-bank mode
- handling of additional option bits (28-31) in FLASH_OPTCR
  in options_read and options_write for F42x/43x/469/479/7xx,
  options bits 0-1 masked out
- check for sensible value of user_options in options_write

- some #defines clarified, non-needed ones removed

- docs updated (options read, options write)

Change-Id: Ie4db80e60baa7d2663e024ab1f278640b1ce901b
Signed-off-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-on: http://openocd.zylin.com/3526
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>