arm64: dts: freescale: sl28: add variant 1

There is a new variant 1 of this board available. It features up to four
SerDes lanes for customer use. Add a new device tree which features just
the basic peripherals. A customer will then have to modify or append to
this device tree.

Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>