stm32l4x.c: Use explicit 64-bit flash access as reference manual implies.

Change-Id: I87b540c1ee7158a9d697e9fbc845a603c6bbe74d
Signed-off-by: Uwe Bonnes <bon@elektron.ikp.physik.tu-darmstadt.de>
Reviewed-on: http://openocd.zylin.com/3139


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