dt-bindings: ata: dwc-ahci: add PHY clocks

Add PHY transmit and receive clocks as described by the
DW SATA AHCI HW manual.

Suggested-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>