dt-bindings: net: fsl: enetc: add the IERB documentation

commit c2ea96eb84e13a5bbd136b3f07e3d76e4c6c1778 from
git://source.codeaurora.org/external/qoriq/qoriq-components/linux



Mention the required compatible string and base address for the
Integrated Endpoint Register Block node.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
(cherry picked from commit 4ac7acc6)
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Meng Li <Meng.Li@windriver.com>