MAINTAINERS: GPIO-INTEL-MID: add maintainer

Assign david.a.cohen@linux.intel.com as maintainer of Intel MID gpio
driver at drivers/gpio/gpio-intel-mid.c.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>