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>

Documentation/devicetree/bindings/pinctrl/samsung-pinctrl.txt

@@ -13,6 +13,7 @@ Required Properties:
   - "samsung,s3c2450-pinctrl": for S3C2450-compatible pin-controller,
   - "samsung,s3c64xx-pinctrl": for S3C64xx-compatible pin-controller,
   - "samsung,s5pv210-pinctrl": for S5PV210-compatible pin-controller,
+  - "samsung,exynos3250-pinctrl": for Exynos3250 compatible pin-controller.
   - "samsung,exynos4210-pinctrl": for Exynos4210 compatible pin-controller.
   - "samsung,exynos4x12-pinctrl": for Exynos4x12 compatible pin-controller.
   - "samsung,exynos5250-pinctrl": for Exynos5250 compatible pin-controller.