net: phy: mscc: make clock-output configurable on vsc85xx

net: phy: mscc: make clock-output configurable on vsc85xx

The vsc8530/8531/8540/8541 phys have a configurable clock output that
can emit 25, 50 and 125 MHz rates, which in turn may be needed for
stable network connections.

This follows a similar change introduced into the Linux kernel at
  https://lore.kernel.org/netdev/20200609133140.1421109-2-heiko@sntech.de

Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>