MAINTAINERS: pinctrl: Add maintainers for pinctrl-single

Otherwise we keep missing patches related to this driver.

Cc: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>