Modify mach-shmobile to only select PINCTRL in case of
ARCH_WANT_OPTIONAL_GPIOLIB is set.

This fixes a build error triggered when adding a new SoC
lacking GPIO software support (ARCH_WANT_OPTIONAL_GPIOLIB=n):

 CC      drivers/tty/vt/keyboard.o
In file included from drivers/pinctrl/core.c:30:0:
include/asm-generic/gpio.h: In function 'gpio_get_value_cansleep':
include/asm-generic/gpio.h:270:2: error: implicit declaration of function '__gpio_get_value'
include/asm-generic/gpio.h: In function 'gpio_set_value_cansleep':
include/asm-generic/gpio.h:276:2: error: implicit declaration of function '__gpio_set_value'
drivers/pinctrl/core.c: In function 'pinctrl_ready_for_gpio_range':
drivers/pinctrl/core.c:297:9: error: implicit declaration of function 'gpio_to_chip'
drivers/pinctrl/core.c:297:27: warning: initialization makes pointer from integer without a cast
drivers/pinctrl/core.c:304:45: error: dereferencing pointer to incomplete type
drivers/pinctrl/core.c:305:26: error: dereferencing pointer to incomplete type
drivers/pinctrl/core.c:305:39: error: dereferencing pointer to incomplete type
make[2]: *** [drivers/pinctrl/core.o] Error 1
make[1]: *** [drivers/pinctrl] Error 2
make[1]: *** Waiting for unfinished jobs....
  LD      drivers/sh/built-in.o
Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>