ARM: dts: add secure firmware support for exynos5420-arndale-octa

Arndale-Octa board is always configured to work with trustzone
firmware binary. Added DTS node entry to enable this support.

Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>