imx8x: Fixup serial driver

Signed-off-by: Lukas Humbel <lukas.humbel@inf.ethz.ch>

serial lpuart driver (reading characters and sending them to the shell)

Signed-off-by: Aya Kayal <akayal@student.ethz.ch>

imx8x: Fix lpuart Mackerel file

Mackerel functions would read the data register when writing it.
Therefore dropping any received character.

Signed-off-by: Daniel Schwyn <daniel.schwyn@inf.ethz.ch>

Add lpuart kernel driver

Signed-off-by: Daniel Schwyn <daniel.schwyn@inf.ethz.ch>