ARM: board-mx51_babbage: Fix UART2 registration

On MX51 Babbage board the RTS/CTS pins are not used on UART2 port.

Fix the registration of this port.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>