ARM: dts: Enable wifi power-on for exynos5250-snow

The Snow board has a MMC/SDIO wifi chip that is always powered but it
needs a power sequence involving a reset (active low) and an enable
(active high) pins. Both pins are marked as active low since the MMC
simple power sequence driver asserts the pins prior to the card power
up procedure and de-asserts the pins after the card has been powered.

So the reset line will be left de-asserted and the enable pin will be
left asserted.

The chip also needs an external 32kHz reference clock to be operational
that is by the MAX77686 PMIC clock.

Add a simple MMC power sequence provider for the wifi MMC/SDIO slot.

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