dt-bindings: reset: meson-axg: fix SPDX license id

As reported, the SPDX license id is not placed correctly and the variant
of the BSD License used should be specified.

Fixes: c1629257 ("dt-bindings: reset: Add bindings for the Meson-AXG SoC Reset Controller")
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Rob Herring <robh@kernel.org>