clk: meson: use SPDX license identifiers consistently

Replace every license notices in drivers/clk/meson by SPDX license
identifiers, as described in license-rules.rst

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Stephen Boyd <sboyd@kernel.org>
Signed-off-by: Jerome Brunet <jbrunet@baylibre.com>