Unlike what was initially claimed in the PHY binding, the interrupt
property of a PHY can be omitted, and the OS will turn to polling instead.

Document that.
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Rob Herring <robh@kernel.org>