arm64: dts: realtek: Change dual-license from MIT to BSD

Move the SPDX-License-Identifier to the top line and update to SPDX 2.0.
While at it, switch from GPLv2+/MIT to GPLv2+/BSD2c before adding more.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>