dt-bindings: net: renesas,etheravb: Fix optional second clock name

If the optional "clock-names" property is present, but the optional TXC
reference clock is not, "make dtbs_check" complains:

    ethernet@e6800000: clock-names: ['fck'] is too short

Fix this by declaring that a single clock name is valid.
While at it, drop the superfluous upper limit on the number of clocks,
as it is implied by the list of descriptions.

Fixes: 6f43735b

 ("dt-bindings: net: renesas,etheravb: Add additional clocks")
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/b3d91c9f70a15792ad19c87e4ea35fc876600fae.1620118901.git.geert+renesas@glider.be
Signed-off-by: Rob Herring <robh@kernel.org>