arm64: dts: freescale: sl28: combine SPI MTD partitions

The upstream port, doesn't really follow the vendor partitioning. The
bootloader partition has one U-Boot FIT image containing all needed
bits and pieces. Even today the bootloader is already larger than the
current "bootloader" partition. Thus, fold all the partitions into one
and keep the environment one. The latter is still valid.
We keep the failsafe partitions because the first half of the SPI flash
is preinstalled by the vendor and immutable.

Fixes: 815364d0

 ("arm64: dts: freescale: add Kontron sl28 support")
Signed-off-by: Michael Walle <michael@walle.cc>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>