stm32: add support for stm32l1x 256k high density single bank devices

Added support for new ST devices in the stm32lx portfolio, with device
id 0x427. These have 256k flash, but in a single bank compared to
device id 0x436 which is a dual bank flash.

Change-Id: Iafdfe990f24bd04b0d6e00385ee70690f3bf8d5f
Signed-off-by: Johan Almquist <johan.almquist@assaabloy.com>
Reviewed-on: http://openocd.zylin.com/1140
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>