DT: phy.txt: Clarify expected compatible values

PHY devices may only list compatibility with clause 22, 45, and if
they need to be more specific, their PHY identifier values. No other
compatible strings are allowed.  Make this clear in the documentation,
and remove examples where make/model compatible strings are listed.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>