powerpc: mpc85xx: Add dummy gpio.h to enable CONFIG_OF_CONTROL

If CONFIG_OF_CONTROL is enabled, lib/fdtdec.c is compiled.
It includes <asm/gpio.h> and then <asm/gpio.h> includes
<asm/arch/gpio.h>.  As a result, all the SoCs that enable
CONFIG_OF_CONTROL must have <asm/arch/gpio.h>.

The right fix would be to split the lib/fdtdec.c to remove
dependency on GPIO.

This commit adds a dummy <asm/arch/gpio.h> to support OF_CONTROL
for mpc85xx platform. A file mpc85xx_gpio.h exists in
arch/powerpc/include/asm. The defintions in that file conflict
with the ones in asm-generic/gpio.h. Hence a dummy header file
has been added. This will be removed after FDT-GPIO stuff is
fixed correctly.

Signed-off-by: Ruchika Gupta <ruchika.gupta@freescale.com>
Reviewed-by: York Sun <yorksun@freescale.com>