arm64: dts: marvell: Enable second SDHCI controller in Armada 37xx

The Armada 37xx SoCs has 2 SDHCI interfaces. This patch adds the second
one.

Moreover, the Armada 37xx DB v2 board populates the 2 SDHCI interfaces.

The second interface is using pluggable module that can either
have an SD connector or eMMC on it.
This patch adds support for SD module in the device DT.

[ gregory.clement@free-electrons.com:
 - Add more detail in commit log
 - Sort the dt node in address order
 - Document the SD slot in the dts ]

Signed-off-by: Konstantin Porotchkin <kostap@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>