Declare a module for evdev and add dependency to ukbd(4) and ums(4)

Prepare for making evdev a module. "Pure" evdev device drivers (like
touchscreen) and evdev itself can be built as a modules regardless of
"options EVDEV" in kernel config. So if people does not require evdev
functionality in hybrid drivers like ums and ukbd they can, for instance,
kldload evdev and utouchscreen to run FreeBSD in kiosk mode.