net: mvneta: update clocks property and document additional clock-names

- interrupts: interrupt for the device

- phy: See ethernet.txt file in the same directory.

- phy-mode: See ethernet.txt file in the same directory

- clocks: a pointer to the reference clock for this device.

- clocks: List of clocks for this device. At least one clock is

  mandatory for the core clock. If several clocks are given, then the

  clock-names property must be used to identify them.

Optional properties:

- tx-csum-limit: maximum mtu supported by port that allow TX checksum.

  Value is presented in bytes. If not used, by default 1600B is set for

  "marvell,armada-370-neta" and 9800B for others.

- clock-names: List of names corresponding to clocks property; shall be

  "core" for core clock and "bus" for the optional bus clock.

Example: