ARM: davinci: defconfig: enable devtmpfs and devtmpfs automount

Since udev-176, udev no longer creates device nodes under /dev
and this has to be managed by the kernel devtmpfs filesystem.

This means that a kernel built with the current defconfigs
will not boot on a system with a recent udev.

Also, enable automatic mounting of /dev for filesystems which
don't manually mount it.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>