arm64: zynqmp: Remove incorrect comment from kv260s

Remove incorrect comment about required nodes by spec. In past gem3 was the
part of SOM specification but it has been revisit by introducting KR260.

Link: https://lore.kernel.org/r/0bd166345ce78097a1ff8d4307545f5026c92267.1704728353.git.michal.simek@amd.com


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