ASoC: Add initial WM8995 driver

The WM8995 is a digital audio hub CODEC designed for smartphones.
The current driver supports most of the basic functionality of the
WM8995.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>