ASoC: Intel: byt-rt5640: Add quirk for Dell Venue 8 Pro tablet

It was found with help of Jan-Michael Brummer <jan.brummer@tabos.org> that
Dell Venue 8 Pro tablet has a digital microphone connected to DMIC2
interface of the RT564x.

This patch adds a DAPM route to DMIC2 and a quirk using it for that tablet.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reported-by: Jan-Michael Brummer <jan.brummer@tabos.org>
ASoC: Intel: byt-rt5640: Set card as fully routed

Although it's not known does current version of byt-rt5640 cover all
possible variants it is better to set the fully_routed flag on in order to
disable unused codecs pins in known machines and get regression from
machines that use different routing than the default one.

Signed-off-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>