arc: add empty asm/processor.h to satisfy compilation of USB code

common/usb.c unconditionally includes <asm/processor.h>
And now to allow USB support on ARC boards we have to have that header.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>

arch/arc/include/asm/processor.h

+/*
+ * Copyright (C) 2015 Synopsys, Inc. All rights reserved.
+ *
+ * SPDX-License-Identifier:	GPL-2.0+
+ */
+
+#ifndef _ASM_ARC_PROCESSOR_H
+#define _ASM_ARC_PROCESSOR_H
+
+/* This file is required by some generic code like USB etc */
+
+#endif /* _ASM_ARC_PROCESSOR_H */