kinetis: ensure flash writes are not truncated

The number if longwords or "sections" (Freescale term) written for a Kinetis
flash write (4, 8, or 16 bytes depending on the part density/granularity) are
now rounded up to ensure there are no truncations when the desired write is
not a multiple of the minimum write size.

Change-Id: I8db40a8769d8ac5393a46cbf4e5ff0df82faf916
Signed-off-by: Christopher Kilgour <techie@whiterocker.com>
Reviewed-on: http://openocd.zylin.com/738


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