stm32l4: support flashing L45x/46x devices

Also fixes incorrect comment about MSI range.

Change-Id: If1339a00e50db44195dfcd5c767ba3f5d9035451
Signed-off-by: Juha Niskanen <juha.niskanen@haltian.com>
Reviewed-on: http://openocd.zylin.com/4122


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>