flash/nor/stm32f2x: fix protection block size for F767 in dual bank mode

flash/nor/stm32f2x: fix protection block size for F767 in dual bank mode

A protection block comprises two adjacent sectors in dual bank mode.
As there are 64 and 128kB sectors joined in blocks 2 and 8, block size
should be computed as a sum of sector sizes.

Change-Id: Ie915df8cf7ca232c4565d7e0c514c8933e71fdfe
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: http://openocd.zylin.com/4271
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>