pinctrl: exynos5440: fix probe failure due to missing pin-list in config nodes

The property 'samsung,exynos5440-pins' is optional in configuration nodes
which are included in the Exynos5440 pin-controller device node. Fix the
incorrect failure in driver probe if 'samsung,exynos5440-pins' property
is not found in the configuration nodes.

Signed-off-by: Thomas Abraham <thomas.ab@samsung.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>