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>

arch/arm/boot/dts/exynos5250-snow.dts

@@ -567,6 +567,7 @@ &mmc_3 {
 	num-slots = <1>;
 	broken-cd;
 	cap-sdio-irq;
+	keep-power-in-suspend;
 	card-detect-delay = <200>;
 	samsung,dw-mshc-ciu-div = <3>;
 	samsung,dw-mshc-sdr-timing = <2 3>;