dt-bindings: net: dwmac: increase 'maxItems' for 'clocks', 'clock-names' properties

This change is needed for some soc based on snps,dwmac, which have
more than 3 clocks.

Signed-off-by: Christophe Roullier <christophe.roullier@st.com>
Signed-off-by: Rob Herring <robh@kernel.org>