phy: miphy365x: Represent each PHY channel as a DT subnode

This has the added advantages of being able to enable/disable each
of the channels as simply as enabling/disabling the DT node.

Suggested-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>