dt-bindings: net: rockchip-dwmac: fix {tx|rx}-delay defaults/range in schema

The range and the defaults are specified in the description instead of
being specified in the schema.
Fix it by adding the default value in the `default` field and specifying
the range as `minimum` and `maximum`.

Fixes: b331b8ef ("dt-bindings: net: convert rockchip-dwmac to json-schema")
Signed-off-by: Eugen Hristev <eugen.hristev@collabora.com>
Signed-off-by: David S. Miller <davem@davemloft.net>