AT91SAM4L: Fix bug when writing to flash at a non-zero offset, add debug

AT91SAM4L: Fix bug when writing to flash at a non-zero offset, add debug
output for flash page writes

The offset wasn't being considered in the "full page" write codepath, so any
writes at an offset were actually written out starting from page 0.

Change-Id: I5e70a1f35f144b3edd1ce6d9df9af9b5da6cf194
Signed-off-by: Angus Gratton <gus@projectgus.com>
Reviewed-on: http://openocd.zylin.com/1965
Tested-by: jenkins
Reviewed-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-by: Paul Fertser <fercerpav@gmail.com>