[ARM] pxa: fix missing of __REG() definition for ac97 registers access

This currently happens for 'drivers/input/touch/mainstone-wm97xx.c'.
Signed-off-by: Eric Miao <eric.miao@marvell.com>

arch/arm/mach-pxa/include/mach/regs-ac97.h

 #ifndef __ASM_ARCH_REGS_AC97_H
 #define __ASM_ARCH_REGS_AC97_H
 
+#include <mach/hardware.h>
+
 /*
  * AC97 Controller registers
  */