ARM: dts: berlin: add the AHCI node for the BG2Q

The BG2Q has an AHCI SATA controller. Add the corresponding nodes
(AHCI, PHY) into its device tree.

Signed-off-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>