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>

Author: Russell King (Oracle)

include/uapi/linux/mdio.h

@@ -125,6 +125,7 @@
 #define MDIO_STAT1_LPOWERABLE		0x0002	/* Low-power ability */
 #define MDIO_STAT1_LSTATUS		BMSR_LSTATUS
 #define MDIO_STAT1_FAULT		0x0080	/* Fault */
+#define MDIO_PCS_STAT1_CLKSTOP_CAP	0x0040
 #define MDIO_AN_STAT1_LPABLE		0x0001	/* Link partner AN ability */
 #define MDIO_AN_STAT1_ABLE		BMSR_ANEGCAPABLE
 #define MDIO_AN_STAT1_RFAULT		BMSR_RFAULT