ENGR00230364 - imx6sl: FEC: fix cycle reboot fail in EVK platform for bootp.

In some imx6sl evk boards, fec cannot work fine while doing
cycle reboot via to execute command "reboot" in kernel.

The root cause: phys clock source is closed when reboot system,
and LAN8720 status machine is in disorder. So it needs to do phy
hardware reset to make phy enter normal state machine.

Signed-off-by: Fugang Duan <B38611@freescale.com>