extcon: arizona: Add defines for microphone detection levels

Improve readability by creating a define for each microphone detection
level.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>