ARM: dts: imx6-tx6: enable support for rtscts on UARTs

Add missing pinctrl for the RTS/CTS lines to uart1 and set the
fsl,uart-has-rtscts property on all UARTs to enable support for HW
handshake.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>