dt-bindings: usb: generic-ehci: Add "companion" entry

"companion" entry is present in "generic.txt" usb binding file. This commit
adds it also in generic-ehci yaml binding.

Signed-off-by: Alexandre Torgue <alexandre.torgue@st.com>
[robh: restrict type to phandle instead of phandle-array]
Signed-off-by: Rob Herring <robh@kernel.org>