Documentation: dt-bindings: Add a new compatible for the Armada 3700

The AHCI interfaces used in the Armada 3700 has a few differences with
the one used in the Armada 38x, so it deserves its own compatible string.
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/ata/ahci-platform.txt

@@ -13,6 +13,7 @@ Required properties:
   - "hisilicon,hisi-ahci"
   - "ibm,476gtr-ahci"
   - "marvell,armada-380-ahci"
+  - "marvell,armada-3700-ahci"
   - "snps,dwc-ahci"
   - "snps,exynos5440-ahci"
   - "snps,spear-ahci"