net: mdio: add definition for clock stop capable bit

Add a definition for the clock stop capable bit in the PCS MMD. This
bit indicates whether the MAC is able to stop the transmit xMII clock
while it is signalling LPI.

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/E1tYADb-0014PV-6T@rmk-PC.armlinux.org.uk


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