ASoC: wm_adsp: Add support for SYSTEM firmware controls

Add support for firmware controls marked SYSTEM. These are
internal to the driver-firmware interface and do not have
a user-accessible ALSA control.

Signed-off-by: Stuart Henderson <stuarth@opensource.wolfsonmicro.com>
Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>