defconfig: fsm9900: Add devtmpfs support

Support for devtmpfs is required on fsm9900 targets.
The hotplug uevent is deprecated.

Change-Id: I9bdff2ffb736c7e059ec76ccae1bdbb473ad7889
Acked-by: Jennifer L. Zenner <jzenner@qti.qualcomm.com>
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>