net: phy: add support for querying PHY clock stop capability

Add support for querying whether the PHY allows the transmit xMII clock
to be stopped while in LPI mode. This will be used by phylink to pass
to the MAC driver so it can configure the generation of the xMII clock
appropriately.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Link: https://patch.msgid.link/E1tYADg-0014Pb-AJ@rmk-PC.armlinux.org.uk


Signed-off-by: Jakub Kicinski <kuba@kernel.org>