irqchip/renesas-irqc: Remove unneeded inclusion of <linux/spinlock.h>

The driver never used spinlocks, and thus does not need to include
<linux/spinlock.h>.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>