ARM: dts: imx7d-pinfunc: add input mux for UART2 RX DTE mode

Add input mux for UART2 RX in DTE mode. This allows to use the pad
UART2_TX_DATA_ALT0 as UART2 RX. This particular input select seems
to be missing in current reference manuals (Rev. B), but when looking
at the tables and other UART input select registers (e.g. UART3) it
seems naturally that this input mux register also has a fourth pad
option for UART2_TX_DATA_ALT0. It has also been proven to be required
to use UART2 in DTE mode and the particular pads on the Colibri iMX7
platform.

Signed-off-by: Stefan Agner <stefan@agner.ch>
Acked-by: Frank Li <Frank.Li@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>