powerpc/mpc85xx: Fix compiling error for common/cmd_gpio.c

To replicate:
1. add to include/configs/p1_p2_rdb_pc.h "#define CONFIG_CMD_GPIO"
2. run `make P1020RDB-PC_defconfig`
3. run CROSS_COMPILE=powerpc-linux- make

and you will get:
common/built-in.o: In function `do_gpio':
u-boot/common/cmd_gpio.c:186: undefined reference to `gpio_request'
u-boot/common/cmd_gpio.c:194: undefined reference to `gpio_direction_input'
u-boot/common/cmd_gpio.c:195: undefined reference to `gpio_get_value'
u-boot/common/cmd_gpio.c:200: undefined reference to `gpio_get_value'
u-boot/common/cmd_gpio.c:203: undefined reference to `gpio_direction_output'
u-boot/common/cmd_gpio.c:209: undefined reference to `gpio_free

Signed-off-by: Michael Durrant <mdurrant@arcturusnetworks.com>
Signed-off-by: Oleksandr G Zhadan <oleks@arcturusnetworks.com>
Reviewed-by: York Sun <yorksun@freescale.com>