UCM: Allow querying of {Supported,Conflicting}Devices

UCM: Allow querying of {Supported,Conflicting}Devices

At any time, one can query:

_supporteddevs/<modifier>|<device>/<verb>
_conflictingdevs/<modifier>|<device>/<verb>

If a verb is current, one can query:

_supporteddevs/<modifier>|<device>
_conflictingdevs/<modifier>|<device>

Note that at most one of the supported/conflicting devs lists has
any entries, and when neither is present, all devices are supported.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>