boards: arm: nxp: convert to DT_NODELABEL usage for UART_MCUX

Towards moving uart mcux driver to DT_INST based, we need to convert
uses of CONFIG_UART_MCUX_[0-9] to use DT_NODELABEL instead.  This way
the pinmux settings are still based on specific instances of the uart.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>
5 files changed