Add a tristate property to advertise desired transmit level.

If the device supports the 2.4 Vpp operating mode for 10BASE-T1L,
as defined in 802.3gc, and the 2.4 Vpp transmit voltage operation
is desired, property should be set to 1. This property is used
to select whether Auto-Negotiation advertises a request to
operate the 10BASE-T1L PHY in increased transmit level mode.

If property is set to 1, the PHY shall advertise a request
to operate the 10BASE-T1L PHY in increased transmit level mode.
If property is set to zero, the PHY shall not advertise
a request to operate the 10BASE-T1L PHY in increased transmit level mode.
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandru Tachici <alexandru.tachici@analog.com>
Signed-off-by: David S. Miller <davem@davemloft.net>