dt-bindings: firmware: xilinx: Describe soc-nvmem subnode

Describe soc-nvmem subnode as the part of firmware node. The name can't be
pure nvmem because dt-schema already defines it as array property that's
why different name should be used.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/24fe6adbf2424360618e8f5ca541ebfd8bb0723e.1706692641.git.michal.simek@amd.com


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