alpha: defconfig: add necessary configs for boot testing

Gentoo's KernelCI will soon boot test alpha kernel and we need
CONFIG_DEVTMPFS=y to be set for that.
Note that CONFIG_DEVTMPFS=y is already necessary for lot of other
distribution/tools like recent udev/systemd.

Signed-off-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: Matt Turner <mattst88@gmail.com>