ARM: mvebu: Update the SDHCI node on Armada 38x

The binding of the armada-380-sdhci has been extended with a new
register in order to be able to use the SDR50 and DDR50 mode. This
commit add the resource associated to this new register for the
Armada 38x.

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