u8500_defconfig: allow creation and mounting of devtmpfs

This patch enables CONFIG_DETTMPFS and CONFIG_DEVTMPFS_MOUNT
flags for u8500 based devices.

In this way it's possible to create a tmpfs/ramfs already in the
early stages of the boot, allowing programs like udev/mdev to
populate the /dev directory.

Signed-off-by: Andi Shyti <andi@etezian.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>