imx: nandbcb: refactor update function

Move code for writing FCB/DBBT pages to a separate function

Signed-off-by: Igor Opaniuk <igor.opaniuk@toradex.com>
Tested-by: Max Krummenacher <max.krummenacher@toradex.com>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@toradex.com>