stm32lx flash: fix flash size for Medium+ devices

stm32lx flash: fix flash size for Medium+ devices

Unfortunately the Medium+ density and 0x436 devices have their F_SIZE register
at a different location: 0x1FF800CC instead of 0x1FF8004C.  Fix this for
the 0x427 Medium+ devices and also the 0x436 devices.  Furthermore, for
0x436 devices the flash size is reported as a 0 or 1 code rather than
the size in Kb.  Please see RM0038 r8 or newer for an explanation, as
noted in the comments.

Change-Id: Ie03b1e119a61f2a854bc2ccc5f90ce3e8852e272
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/1522
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>