dt-bindings: usb: Bring back phy-names

While the original bindings that were superseeded by the YAML schemas
didn't mention that phy-names was needed, it turns out that phy-names is
required if phys is set according to phy/phy-bindings.txt.

Let's add back those properties.

Fixes: 14ec072a ("dt-bindings: usb: Convert USB HCD generic binding to YAML")
Fixes: c93bcace ("dt-bindings: usb: Convert the generic OHCI binding to YAML")
Fixes: c3e2485d

 ("dt-bindings: usb: Convert the generic EHCI binding to YAML")
Reported-by: Emmanuel Vadot <manu@bidouilliste.com>
Signed-off-by: Maxime Ripard <mripard@kernel.org>
Link: https://lore.kernel.org/r/20191002112651.100504-2-mripard@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>