clk: mvebu: armada-39x: add clk description for supported interfaces

Both SATA and second USB3.0 interface are supported in Armada-39x SoC
family. Add necessary clk description, so both xhci and sata drivers
can be correctly initialized.

The binding documentation has also been updated accordingly.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>