arm64: dts: imx8mm-venice-gw7904: enable UART1 hardware flow control

On the GW7904 uart1 can use UART3_TX/RX for hardware flow control
which was the desired default configuration. Remove uart3 and configure
uart1 for hardware flow control.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>