dt-bindings: net: renesas,etheravb: Add additional clocks

The AVB driver assumes there is an external crystal, but it could
be clocked by other means.  In order to enable a programmable
clock, it needs to be added to the clocks list and enabled in the
driver.  Since there currently only one clock, there is no
clock-names list either.

Update bindings to add the additional optional clock, and explicitly
name both of them.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Sergei Shtylyov <sergei.shtylyov@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>