Add libdm and dmctl support for dm-user targets

dm-user is very much a WIP and while this may change (we need to figure out how
to get credentials to the dm-user daemon, for example) it seems like a somewhat
safe bet to assume that the behavior will at least include a start/end range.

Test: I just ran "dmctl create palmer user 0 1024", which created the device.
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>
Change-Id: Ic5f84de6a4f09bf906246035d450edd637cc38ed