ARM: dts: Use active low flag for gpio-keys on Origen

Commit f447ed8b ("gpio: samsung: add flags specifier to
device-tree binding") adds a flag to represent active low state
for gpio line. Since gpio-keys on Origen board are active low,
using this flag to represent the same.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Thomas Abraham <thomas.abraham@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>