mfd: sec: Drop support for board files and require devicetree

The Samsung PMIC drivers since long time are used only on devicetree
platforms (Samsung Exynos) and there are no users with board files.

Drop the support for board files entirely and depend on OF for matching.

Suggested-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>