ARM: mx5: Fix MX53 flexcan2 clock

The second FlexCAN port uses different clock than the first one, configure
correct clock to prevent hanging of the system during bringing up of the port.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>