Add SDHCI node for supporting the micro SD card. On Trats2 board the
SDHCI does not use sd2_cd pins (gpk2-2) for card detect but gpx3-4
instead.

Power is supplied from LDO21 regulator which in off state is controlled
by external GPIO pin.
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>