ASoC: wm8985: add register definitions for WM8758

The WM8758 chip is almost identical to WM8985 with the difference that it
doesn't feature the AUX input. This patch adds the register definitions
for WM8758 specific bit fields to the header file.

Signed-off-by: Petr Kulhavy <petr@barix.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>