clocksource/drivers/timer-sun4i: Convert to SPDX identifier

The license information clearly states GPL version 2 only. The extra text
which excludes warranties is an excerpt of the corresponding GPLv2 clause
11.

So the SPDX identifier covers it completely.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Chen-Yu Tsai <wens@csie.org>
Cc: Jernej Skrabec <jernej.skrabec@gmail.com>
Cc: Samuel Holland <samuel@sholland.org>
Cc: linux-sunxi@lists.linux.dev
Link: https://lore.kernel.org/r/20220510171254.908144392@linutronix.de


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>