clocksource/drivers/armada-370-xp: 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: Gregory Clement <gregory.clement@free-electrons.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20220510171254.592781786@linutronix.de


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