doc: bindings: add bindings document for PHY nodes

It defines that PHY nodes must be children on MDIO bus nodes and defines
the only required property in U-Boot, reg.  This property along with the
example provided are copied over from Linux.

Signed-off-by: Alex Marginean <alexandru.marginean@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>