ARM: mx23: rename mx23 auart irq definition to align with mx28

i.MX23 Reference Manual names auart irq differently from i.MX28.
This patch is to align the naming with mx28, so that some device
registration codes can be shared.

Signed-off-by: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>