dt-bindings: gpio: samsung: drop unused bindings

The commit 6c56c6cd ("gpio: samsung: Drop support for Exynos SoCs")
removed support for the Samsung Exynos SoC in lrgacy GPIO driver, since
it was moved to new pinctrl driver.  Remove old, unused bindings.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>