irqchip/stm32: Fix copyright

Uniformize STMicroelectronics copyrights header and add SPDX identifier

CC: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@st.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Alexandre TORGUE <alexandre.torgue@st.com>
Acked-by: Maxime Coquelin <mcoquelin.stm32@gmail.com>
Cc: jason@lakedaemon.net
Cc: marc.zyngier@arm.com
Cc: linux-arm-kernel@lists.infradead.org
Link: https://lkml.kernel.org/r/20171130084500.23439-1-benjamin.gaignard@st.com