mxc_serial replace platform specific clock

remove ifdef'd clock selection code from
serial_mxc.c and replace with call to imx_get_uartclk

Add definitions for imx_get_uartclk to imx31 and imx27
include files.

This makes it easier to add new imx platforms.

Signed-off-by: John Rigby <jcrigby@gmail.com>
diff --git a/include/asm-arm/arch-mx27/clock.h b/include/asm-arm/arch-mx27/clock.h
index 5fc75c5..472e8f2 100644
--- a/include/asm-arm/arch-mx27/clock.h
+++ b/include/asm-arm/arch-mx27/clock.h
@@ -36,4 +36,6 @@
 ulong imx_get_perclk3(void);
 ulong imx_get_ahbclk(void);
 
+#define imx_get_uartclk imx_get_perclk1
+
 #endif /* __ASM_ARCH_CLOCK_H */