ARM: EXYNOS: Fix compilation warning

of_get_flat_dt_prop return type is now const.
Fixes the following compilation warning introduced by commit 9d0c4dfe
("of/fdt: update of_get_flat_dt_prop in prep for libfdt")

arch/arm/mach-exynos/exynos.c:259:6: warning:
assignment discards ‘const’ qualifier from pointer target type [enabled by default]

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Reviewed-by: Tushar Behera <tushar.behera@linaro.org>
Cc: Rob Herring <robh@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>