ENGR00210654 - MSL : fix NFS boot fails issue in sometime

ENGR00210654 - MSL : fix NFS boot fails issue in sometime

- MX6 sololite cpu board NFS boot fails in sometimes, because MAC
  cannot get any packets while sending DHCP to require IP. The
  reproduce rate is 10%.
- Lan8720 phy enter a unexpected status, and need software reset
  phy before transmition.
- Do some below overnight tests after add the changes, no NFS
  boot issue found.
  	1. Kernel boot from MMC, rootfs mount from NFS.
	2. Kernel boot from tftp, rootfs mount form NFS.

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