x86: Add intr_enable_src()

Function to enable specific IRQ source.

This will be used by the s2idle code to enable just SCIs on x86 to break
the CPU out of idle.

Reviewed by:	olce
Approved by:	olce
Sponsored by:	The FreeBSD Foundation
Differential Revision:	https://reviews.freebsd.org/D48734

sys/x86/include/intr_machdep.h

@@ -151,6 +151,7 @@ int	intr_register_source(struct intsrc *isrc);
 int	intr_remove_handler(void *cookie);
 void	intr_resume(bool suspend_cancelled);
 void	intr_suspend(void);
+void	intr_enable_src(u_int irq);
 void	intr_reprogram(void);
 void	intrcnt_add(const char *name, u_long **countp);
 void	nexus_add_irq(u_long irq);

sys/x86/x86/intr_machdep.c

@@ -405,6 +405,15 @@ intr_suspend(void)
 	mtx_unlock(&intrpic_lock);
 }
 
+void
+intr_enable_src(u_int irq)
+{
+	struct intsrc *is;
+
+	is = interrupt_sources[irq];
+	is->is_pic->pic_enable_source(is);
+}
+
 static int
 intr_assign_cpu(void *arg, int cpu)
 {