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>

include/asm-arm/arch-mx27/clock.h

@@ -36,4 +36,6 @@ ulong imx_get_perclk2(void);
 ulong imx_get_perclk3(void);
 ulong imx_get_ahbclk(void);
 
+#define imx_get_uartclk imx_get_perclk1
+
 #endif /* __ASM_ARCH_CLOCK_H */

include/asm-arm/arch-mx31/mx31.h

@@ -25,6 +25,7 @@
 #define __ASM_ARCH_MX31_H
 
 extern u32 mx31_get_ipg_clk(void);
+#define imx_get_uartclk mx31_get_ipg_clk
 extern void mx31_gpio_mux(unsigned long mode);
 
 enum mx31_gpio_direction {