MAINTAINERS: Add binding docs for Cirrus Logic/Wolfson Arizona devices

Add device tree binding documentation for the Arizona drivers
to the MAINTAINERS file.

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