arm: multi_v7_defconfig: add i.MX options and NFS root

- Add i.MX serial console support. This gives us the boot messages on
  UART on e.g. the i.MX6Q sabre sd platform.
- Add the necessary config options, to allow booting with NFS root on
  an i.MX6 sabre sd.
- Add Freescale LPUART serial console support. This gives us the boot
  messages on UART on e.g. the Vybrid VF610 Tower board.

Signed-off-by: Vincent Stehlé <vincent.stehle@freescale.com>
Cc: Russell King <linux@arm.linux.org.uk>
[olof: squashed three commits down to one]
Signed-off-by: Olof Johansson <olof@lixom.net>