dpll: add frequency monitoring support

dpll: add frequency monitoring support

Add support for the new frequency monitoring feature from the kernel
patch series "dpll: add actual frequency monitoring feature". This
includes:

- DPLL_A_FREQUENCY_MONITOR device attribute (enable/disable)
- DPLL_A_PIN_MEASURED_FREQUENCY pin attribute displayed as fractional Hz
  using DPLL_PR_MEASURED_FREQUENCY macro (kernel reports in mHz)
- device set: frequency-monitor { enable | disable }
- Refactor phase-offset-monitor to use new dpll_parse_attr_feature_state
  helper shared with frequency-monitor
- Update man page and bash-completion

Reviewed-by: Petr Oros <poros@redhat.com>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Signed-off-by: David Ahern <dsahern@kernel.org>