test: dm_mdio: add a 2nd register to the emulated PHY

This 2nd register is used by the follow-up MDIO MUX test.

Signed-off-by: Alex Marginean <alexm.osslist@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>