dt-bindings: phy: Add `tx-p2p-microvolt` property binding

Common PHYs and network PCSes often have the possibility to specify
peak-to-peak voltage on the differential pair - the default voltage
sometimes needs to be changed for a particular board.

Add properties `tx-p2p-microvolt` and `tx-p2p-microvolt-names` for this
purpose. The second property is needed to specify the mode for the
corresponding voltage in the `tx-p2p-microvolt` property, if the voltage
is to be used only for speficic mode. More voltage-mode pairs can be
specified.

Example usage with only one voltage (it will be used for all supported
PHY modes, the `tx-p2p-microvolt-names` property is not needed in this
case):

  tx-p2p-microvolt = <915000>;

Example usage with voltages for multiple modes:

  tx-p2p-microvolt = <915000>, <1100000>, <1200000>;
  tx-p2p-microvolt-names = "2500base-x", "usb", "pcie";

Add these properties into a separate file phy/transmit-amplitude.yaml,
which should be referenced by any binding that uses it.

Signed-off-by: Marek Behún <kabel@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>