stm32: add support for the STM32Lx 384kb dual bank flash

stm32: add support for the STM32Lx 384kb dual bank flash

This update adds support for the STM32Lx 384kb dual bank flash. Previously there was a problem when writing an
image that was larger than 192Kb. That lead to openocd printing out two error messages like
"Error: access denied / write protected" and "Error: invalid program address". The reason was that the stm32lx
driver tried to write half pages which overlapped into the next flash bank.
A new configuration file stm32lx_dual_bank.cfg can be used for stm32lx chips with dual bank flash (256kb or 384kb devices).
A sanity check was added for probed flash size values to fix the issue seen on some ST samples that answered incorrectly.

Change-Id: I69e25131983d88613be8606b438f98870c5f1e52
Signed-off-by: Johan Almquist <johan.almquist@assaabloy.com>
Reviewed-on: http://openocd.zylin.com/1125
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>