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>

arch/alpha/configs/defconfig

@@ -70,3 +70,4 @@ CONFIG_DEBUG_INFO=y
 CONFIG_ALPHA_LEGACY_START_ADDRESS=y
 CONFIG_MATHEMU=y
 CONFIG_CRYPTO_HMAC=y
+CONFIG_DEVTMPFS=y