vexpress64: juno: use /dev/sda2

This patch changes the default "root=" parameter to "/dev/sda2".

Many linux based distros use /dev/sda1 for their boot partition; this is
often not a rootfs that can be used by the "root=" parameter.

Linaro images use /dev/sda1 as a boot partition, although this of a
different nature to a distro image.  Linaro uses /dev/sda2 for the rootfs
partition.

Signed-off-by: Ryan Harkin <ryan.harkin@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>