ARM: make struct machine_desc definition coherent with its comment

As mentioned in the comment right at the top, the first four fields
are directly accessed by assembly code in head.S.  Move nr_irqs so the
comment is true again.

Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>

arch/arm/include/asm/mach/arch.h

@@ -20,14 +20,15 @@ struct machine_desc {
 	 * by assembler code in head.S, head-common.S
 	 */
 	unsigned int		nr;		/* architecture number	*/
-	unsigned int		nr_irqs;	/* number of IRQs */
 	unsigned int		phys_io;	/* start of physical io	*/
 	unsigned int		io_pg_offst;	/* byte offset for io 
 						 * page tabe entry	*/
 
 	const char		*name;		/* architecture name	*/
 	unsigned long		boot_params;	/* tagged list		*/
 
+	unsigned int		nr_irqs;	/* number of IRQs */
+
 	unsigned int		video_start;	/* start of video RAM	*/
 	unsigned int		video_end;	/* end of video RAM	*/