ARM: dts: imx7s-warp: enable uart6 on mikroBus

This patch configures the board to output UART6 to the mikroBus.

The NXP WaRP7 board can route UART6 to the mikroBus, where an RS232
click board can be plugged in to get an additional serial port.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Rui Miguel Silva <rui.silva@linaro.org>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>