net: xpcs: add function to configure EEE clock multiplying factor

net: xpcs: add function to configure EEE clock multiplying factor

Add a function to separate out the EEE clock multiplying factor. This
will be called by the stmmac driver to configure this value.

It would have been better had the driver used the CLK API to retrieve
this clock, get its rate and calculate the appropriate multiplier, but
that door has closed.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1thRQ8-003w70-VT@rmk-PC.armlinux.org.uk
Signed-off-by: Jakub Kicinski <kuba@kernel.org>