The DK2 appears to be just the DK1 with a display, thus factor out the
barebox-specifics into a new stm32mp157a-dk1.dtsi, which we can include
in both device trees after including the upstream device tree.

As the updated device tree introduced new nodes, the now duplicates
(&ethernet0, &uart4, /chosen and /aliases) are dropped from the barebox
device as part of this commit as well.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>