arm64: dts: fsl-ls1028a-kontron-sl28: combine unused partitions

The failsafe partitions for the DP firmware and for AT-F are unused. If
AT-F will ever be supported in the failsafe mode, then it will be a FIT
image. Thus fold the unused partitions into the failsafe bootloader one
to have enough storage if the bootloader image will grow.

While at it, remove the reserved partition. It served no purpose other
than having no hole in the map.

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