UCM: get: Implement 'exact' value retrieval

UCM: get: Implement 'exact' value retrieval

[=]<NAME>[/[<modifier>|</device>][/<verb>]]
- value identifier <NAME>
- Search starts at given modifier or device if any,
    else at a verb
- Search starts at given verb if any,
    else current verb
- Searches modifier/device, then verb, then defaults
- Specify a leading "=" to search only the exact
  device/modifier/verb specified, and not search
  through each object in turn.
- Examples:
    "PlaybackPCM/Play Music"
    "CapturePCM/SPDIF"
  From ValueDefaults only:
    "=Variable"
  From current active verb:
    "=Variable//"
  From verb "Verb":
    "=Variable//Verb"
  From "Modifier" in current active verb:
    "=Variable/Modifier/"
  From "Modifier" in "Verb":
    "=Variable/Modifier/Verb"

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