phy: berlin-sata: Document BG2 compatible

Berlin BG2 SATA PHY is slightly different from currently supported
BG2Q SATA PHY. Document the new compatible for BG2's PHY.

Acked-by: Antoine Ténart <antoine.tenart@free-electrons.com>
Signed-off-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>