gpio: mc9s08dz60: Fix build error if I2C=m

Make GPIO_MC9S08DZ60 depend on I2C=y, this fixes below build error:

  LD      init/built-in.o
drivers/built-in.o: In function `mc9s08dz60_get_value':
clk-fixed-factor.c:(.text+0x7214): undefined reference to `i2c_smbus_read_byte_data'
drivers/built-in.o: In function `mc9s08dz60_set':
clk-fixed-factor.c:(.text+0x727c): undefined reference to `i2c_smbus_read_byte_data'
clk-fixed-factor.c:(.text+0x72bc): undefined reference to `i2c_smbus_write_byte_data'
drivers/built-in.o: In function `mc9s08dz60_i2c_driver_init':
clk-fixed-factor.c:(.init.text+0x290): undefined reference to `i2c_register_driver'
drivers/built-in.o: In function `mc9s08dz60_i2c_driver_exit':
clk-fixed-factor.c:(.exit.text+0x2c): undefined reference to `i2c_del_driver'
make: *** [vmlinux] Error 1

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>