pinctrl: samsung: Document Exynos3250 SoC support

Add missing compatible id for Exynos3250 SoC to device tree docs.
Exynos pin control driver supports it since commit d97f5b98
("pinctrl: exynos: Add driver data for Exynos3250").

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Tomasz Figa <tomasz.figa@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>