MX31: Improve readability for reset cause

Currently the reset cause is printed like:
CPU:   Freescale i.MX31 rev 2.0 at 531 MHz.Reset cause: POR

Improve readability by adding a new line like it is done on other i.MX boards.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>