amd-xgbe: Convert to SPDX identifier

amd-xgbe: Convert to SPDX identifier

Use SPDX-License-Identifier accross all the files of the xgbe driver to
ensure compliance with Linux kernel standards, thus removing the
boiler-plate template license text.

Signed-off-by: Raju Rangoju <Raju.Rangoju@amd.com>
Acked-by: Shyam Sundar S K <Shyam-sundar.S-k@amd.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250407102913.3063691-1-Raju.Rangoju@amd.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>