ASoC: Enable rbtree compression for WM8994/58 register cache

The WM8994 and WM8958 register map is relatively sparse so benefits
from compression. The rbtree compression gives better results than
LZO for both memory and CPU consumption on a map as sparse as this.

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