ARM: Make multi_v7_defconfig usable on hardware

This updates multi_v7_defconfig to boot on at least Tegra2 Seaboard and
Panda ES. I lack working hardware for most of the platforms that were
enabled to compare with, so I could only verify those two for now. Still,
I enabled all of the available ones for build coverage purposes.

Beyond base platforms, things like PRINTK_TIME, filesystems, MMC, USB,
INET and UNIX networking was enabled. I'm sure we'll keep adding more
over time, but this gets us off the ground for semi-regular boot testing.

Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>