net: stmmac: deprecate "snps,en-tx-lpi-clockgating" property

net: stmmac: deprecate "snps,en-tx-lpi-clockgating" property

Whether the MII transmit clock can be stopped is primarily a property
of the PHY (there is a capability bit that should be checked first.)
Whether the MAC is capable of stopping the transmit clock is a separate
issue, but this is already handled by the core DesignWare MAC code.

Therefore, snps,en-tx-lpi-clockgating is technically incorrect, and
this commit adds a warning should a DT be encountered with the property
present.

However, we keep backwards compatibility.

Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Link: https://patch.msgid.link/E1tsIUK-005vGk-H7@rmk-PC.armlinux.org.uk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>