net: stmmac: dwc-qos: use PHY clock-stop capability

Use the PHY clock-stop capability when programming the MAC LPI mode,
which allows the transmit clock to the PHY to be gated. Tested on the
Jetson Xavier NX platform.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://patch.msgid.link/E1u4zi1-000xHh-57@rmk-PC.armlinux.org.uk


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