hummingboard: Enable sgtl5000

This patch is based on linux-linaro-stable-mx6.
(branch:linux-linaro-lsk-v3.14-mx6/commit:dd4bf6a458a9f96102d2e77f16c93192259a5b5a)

This patch does not adversely affect BD-SL-i.MX6 and Wandboard.