ARM: dts: Add sound node for exynos5250-snow board

The audio codec on Snow board, MAX98095 is connected on I2C7 bus.
Also it requires the GPX1-7 line to be pulled up.

Updated Snow DTS file to incorporate above changes and added a
sound node to instantiate the I2S-based sound card.

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