ARM: mmp: fix build error on gpio

Parameters of GPIO_REG() should be assigned as volatile.

arch/arm/plat-pxa/include/plat/gpio.h: In function ‘gpio_get_value’:
arch/arm/plat-pxa/include/plat/gpio.h:12:21: error: invalid operands to
binary & (have ‘void *’ and ‘int’)
arch/arm/plat-pxa/include/plat/gpio.h: In function ‘gpio_set_value’:
arch/arm/plat-pxa/include/plat/gpio.h:21:4: error: lvalue required as
left operand of assignment
arch/arm/plat-pxa/include/plat/gpio.h:23:4: error: lvalue required as
left operand of assignment

Signed-off-by: Haojian Zhuang <haojian.zhuang@marvell.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>

arch/arm/mach-mmp/include/mach/gpio-pxa.h

@@ -7,7 +7,7 @@
 #define GPIO_REGS_VIRT	(APB_VIRT_BASE + 0x19000)
 
 #define BANK_OFF(n)	(((n) < 3) ? (n) << 2 : 0x100 + (((n) - 3) << 2))
-#define GPIO_REG(x)	(GPIO_REGS_VIRT + (x))
+#define GPIO_REG(x)	(*(volatile u32 *)(GPIO_REGS_VIRT + (x)))
 
 #define NR_BUILTIN_GPIO		IRQ_GPIO_NUM