ARM: dts: imx6sx: Improve UART pins macro defines

Add DCE/DTE to UART pins macro defines to distinguish the
DCE and DTE functions, keep old defines at the end of file
for some time to make it backward compatible.
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>