Add GPO0 and GPO1 (General Purpose Outputs) controls to mixer.
These can be used on some cards to control amplifier mute (seen in ES1868
datasheet) or additional onboard chips such as QX2130 QXpander processor.

These GPOs are present on ES1868, ES1869, ES1887 and ES1888 chips.

Tested on ES1868 with QX2130.
Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>