pinctrl/samsung: don't allow enabling pinctrl-samsung standalone

The main samsung pinctrl module references the specific exynos4210
pinctrl driver, which selects the main driver in Kconfig.

Making the main driver a silent "bool" option avoid this potential
build error if CONFIG_PINCTRL_SAMSUNG=y && CONFIG_PINCTRL_EXYNOS4=n:

drivers/built-in.o:(.rodata+0x4e4): undefined reference to `exynos4210_pin_ctrl'

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Tomasz Figa <t.figa@samsung.com>
Cc: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>