drivers: serial: mcux: enforce an error-free header inclusion

We need to change the order of inclusions in
uart_mcux_lpuart.c, to avoid build errors. This
is required since the driver structures contain
a field named DATA, which is also a macro defined
in the linker script.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>