net: phy: realtek: Support external PHY clock

[Detlev Casanova]

Hi, thank you for your comments ! HEre are the changes that were
discussed.

Some PHYs can use an external clock that must be enabled before probing
it.

Changes since v1:
 * Remove the clock name as it is not guaranteed to be identical across
   different PHYs
 * Disable/Enable the clock when suspending/resuming