board: phytec: phycore_imx8mp: Change debug UART

With the first redesign the debug UART had changed from
UART2 to UART1.
As the first hardware revision is considered as alpha and
will not be supported in future. The old setup will not
be preserved.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>