clocksource/drivers/pistachio: Convert to SPDX identifier

The licensing text references explicitely the COPYING file in the kernel
base directory, which is clearly GPL version 2 only.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Ezequiel Garcia <ezequiel.garcia@imgtec.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20220510171254.843410802@linutronix.de


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