arm64: zynqmp: Align nvmem-fw node with dt-schema

Node name has to be renamed to be aligned with dt-schema and also
xlnx,zynqmp-nvmem-fw switched to fixed-layout.

Signed-off-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/32899b20c1e282aab16c32074b1c9a3f45f6dac8.1706791116.git.michal.simek@amd.com