arm64: zynqmp: Add description for SOM/Kria boards

The patch contains several DT files for SOM platform.
Carrier card is sck-kv (KV260) revA/B. SMK-K26 is description for starter
kit which doesn't have EMMC populated. And SM-K26 is full som with EMMC.

Files are divided in this way to make sure that SOM can be plugged to
different carrier card and all peripherals on SOM (or defined by a spec) can
be used by U-Boot. Full DT for SOM+CC can be merged together as overlays.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>