genirq/irq_sim: Use the SPDX license identifier in the header

Use C-style comment for the identifier as per
Documentation/process/license-rules.rst and remove the license boilerplate.

Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20180426200747.8344-2-brgl@bgdev.pl