ARM: dts: mvebu: armada-39x: update the SDHCI node on Armada 39x

Commit 1140011e

 ("mmc: sdhci-pxav3: Modify clock settings
for the SDR50 and DDR50 modes") has extended the Device Tree
binding used to describe PXAv3 SDHCI controllers in order to be
able to use the SDR50 and DDR50 modes.

This commit updates the Device Tree description of the Armada
39x SDHCI controller in other to take advantage of this
functionality.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>