ARM: dts: Remove leftover spi0 node for smdk5250

Now that the SPI controllers are disabled by default for Exynos5250
there is no need to explicitly disable them in individual board files.
This hunk appears not to have been merged when doing the original
conversion, add it now.

Signed-off-by: Mark Brown <broonie@linaro.org>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>