dts: rdb2/3: enable pfe1 mdio node

Now the SDK BSP34 has been officially released, and the pfe1 port on
RDB2 and RDB3 boards is enabled by default, so drop the disable status
to keep align with the kernel driver.

Signed-off-by: Zhantao Tang <zhantao.tang@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>