pinctrl: spear: guard sub-directory with CONFIG_PINCTRL_SPEAR

CONFIG_PINCTRL_SPEAR is more suitable than CONFIG_PLAT_SPEAR
to guard the drivers/pinctrl/spear/ directory.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>