Now that we have aliases for I2C and SPI in ste-dbx5x0.dtsi,
it does not make much sense to keep only the aliases for UART
separately in each board device tree.

Considering that all boards set the same aliases for the serial
ports there is no reason to keep them separated either.

Move them to ste-dbx5x0.dtsi and remove the aliases from the
board-specific device tree parts.
Signed-off-by: Stephan Gerhold <stephan@gerhold.net>
Link: https://lore.kernel.org/r/20191125170428.76069-3-stephan@gerhold.netSigned-off-by: Linus Walleij <linus.walleij@linaro.org>