dt-bindings: deprecate "snps,en-tx-lpi-clockgating" property

dt-bindings: 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, so this
commit deprecates the property in the binding.

Acked-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://patch.msgid.link/E1tsIUF-005vGd-C5@rmk-PC.armlinux.org.uk
Signed-off-by: Paolo Abeni <pabeni@redhat.com>