arm64: dts: ls1028a: sl28: split variant 3/ads2 carrier

arm64: dts: ls1028a: sl28: split variant 3/ads2 carrier

The devicetree files can be (re-)used in u-boot now, they are imported
on a regular basis (see OF_UPSTREAM option) there. Up until now, it
didn't matter for linux and there was just a combined devicetree
"-var3-ads2" (with ads2 being the carrier board). But if the devicetree
files are now reused in u-boot, we need to have an individual "-var3"
variant, because the bootloader is just using the bare "varN" devicetree
files. Split the "var3" off of the "-var3-ads2" devicetree.

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