ARM: DT: STi: STiH407: Add DT node for MiPHY28lp

The MiPHY28lp is a Generic PHY which can serve various SATA, PCIe or
USB3 devices. The two first ports can be use for either; both SATA, both
PCIe or one of each in any configuration.
The Third port is only for USB3.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>