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>