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>

arch/powerpc/include/asm/arch-mpc85xx/gpio.h

@@ -12,4 +12,6 @@
 #ifndef __ASM_ARCH_MX85XX_GPIO_H
 #define __ASM_ARCH_MX85XX_GPIO_H
 
+#include <asm/mpc85xx_gpio.h>
+
 #endif

arch/powerpc/include/asm/mpc85xx_gpio.h

@@ -72,9 +72,10 @@ static inline int gpio_request(unsigned gpio, const char *label)
 	return 0;
 }
 
-static inline void gpio_free(unsigned gpio)
+static inline int gpio_free(unsigned gpio)
 {
 	/* Compatibility shim */
+	return 0;
 }
 
 static inline int gpio_direction_input(unsigned gpio)
@@ -97,12 +98,13 @@ static inline int gpio_get_value(unsigned gpio)
 	return !!mpc85xx_gpio_get(1U << gpio);
 }
 
-static inline void gpio_set_value(unsigned gpio, int value)
+static inline int gpio_set_value(unsigned gpio, int value)
 {
 	if (value)
 		mpc85xx_gpio_set_high(1U << gpio);
 	else
 		mpc85xx_gpio_set_low(1U << gpio);
+	return 0;
 }
 
 static inline int gpio_is_valid(int gpio)