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>