Some clock sources have 3-bit muxes in bits 31:29. Implement core
support for this mux field.

Signed-off-by: Tom Warren <twarren@nvidia.com>
[swarren, extracted from a larger patch by Tom]
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Tom Warren <twarren@nvidia.com>