common: Move interrupt functions into a new header

These functions do not use driver model but are fairly widely used in
U-Boot. But it is not clear that they will use driver model anytime soon,
so we don't want to label them as 'legacy'.

Move them to a new irq_func.h header file. Avoid the name 'irq.h' since it
is widely used in U-Boot already.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>
diff --git a/arch/sandbox/lib/interrupts.c b/arch/sandbox/lib/interrupts.c
index 4ddcd37..21f761a 100644
--- a/arch/sandbox/lib/interrupts.c
+++ b/arch/sandbox/lib/interrupts.c
@@ -6,6 +6,7 @@
  */
 
 #include <common.h>
+#include <irq_func.h>
 
 int interrupt_init(void)
 {