MAINTAINERS: Add entry for Intel pin controller drivers

Add MAINTAINERS entry for Intel pin controller drivers. I will be
maintaining them with Heikki, who kindly promised to help me with this.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>