riscv: print the unexpected interrupt cause

This has been helpful when debugging my pending nommu port.
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Nick Kossifidis <mick@ics.forth.gr>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>

arch/riscv/kernel/irq.c

@@ -54,7 +54,8 @@ asmlinkage void __irq_entry do_IRQ(struct pt_regs *regs)
 		handle_arch_irq(regs);
 		break;
 	default:
-		panic("unexpected interrupt cause");
+		pr_alert("unexpected interrupt cause 0x%lx", regs->scause);
+		BUG();
 	}
 	irq_exit();