dt-bindings: net: snps, dwmac: update reg minItems maxItems

The Amlogic Meson DWMAC glue bindings needs a second reg cells for the
glue registers, thus update the reg minItems/maxItems to allow more
than a single reg cell.

Also update the allwinner,sun7i-a20-gmac.yaml derivative schema to specify
maxItems to 1.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>