ARM: dts: imx53: add alternative UART2 configuration

UART2 on EIM_D26 - EIM_D29 pins supports interchanging RXD/TXD pins
and RTS/CTS pins.
One board using these alternate settings is Beckhoff CX9020. Add the
alternative configuration here, to make it available to others, too.

Signed-off-by: Patrick Bruenn <p.bruenn@beckhoff.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>