pinctrl: samsung: use CONFIG_PINCTRL_SAMSUNG symbol in makefile

Samsung Exynos7 is a ARM64bit processor. Which does not select
the CONFIG_PLAT_SAMSUNG symbol. CONFIG_PINCTRL_SAMSUNG is being
selected for both PLAT_SAMSUNG and ARCH_EXYNOS7 symbols.

This patch modifes the pinctrl/Makefile to use
CONFIG_PINCTRL_SAMSUNG symbol to compile the pinctrl/samsung/*.c

Signed-off-by: Naveen Krishna Chatradhi <ch.naveen@samsung.com>
Cc: Tomasz Figa <t.figa@samsung.com>
Cc: linus.walleij@linaro.org
Cc: Thomas Abraham <thomas.ab@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>