ARM: dts: Add keep-power-in-suspend to WiFi SDIO node for exynos5250-snow

The Marvell mwifiex driver prevents the system to enter into a suspend
state if the card power is not preserved during a suspend/resume cycle.

So Suspend-to-RAM and Suspend-to-idle are failing on Exynos5250 Snow.

Add the keep-power-in-suspend Power Management property to the SDIO/MMC
node so the mwifiex suspend handler doesn't fail and the system is able
to enter into a suspend state.

Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Reviewed-by: Doug Anderson <dianders@chromium.org>
Signed-off-by: Kukjin Kim <kgene@kernel.org>