arm64: zynqmp: Add description for nvmem efuse layout

Based on discussion with DT folks at link below there is not going to be
any name restrictions for child names. That's why add description for
current nvmem layout.

Link: https://lore.kernel.org/lkml/20231013101450.573-3-praveen.teja.kundanala@amd.com/


Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/af81299cafc2bd13ed30dcd69bdf6efb5fbb7f68.1698840373.git.michal.simek@amd.com