drivers: uart_mcux: Implement uart_config_get API

When uart_configure support was added uart_config_get was not
implemented.  Move the 'struct uart_config' to uart_mcux_data lets us
save the uart_config settings and easily return it in
uart_mcux_config_get.

Fixes: #27420

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