net: phy: realtek: Support external PHY clock

From: Detlev Casanova
Date: Mon Jun 05 2023 - 11:20:16 EST


Some PHYs can use an external clock that must be enabled before
communicating with them.

Changes since v2:
* Reword documentation commit message

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