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>
Signed-off-by: Mark Brown <broonie@kernel.org>