MAINTAINERS: Add my hexagon git tree

Add my git tree for hexagon.  Note that the branch is "hex-next" and not
"hex.next" as had been used previously.  But I'll keep the "hex.next" branch
in sync with "hex-next" until this commit lands to avoid confusion.

Signed-off-by: Brian Cain <bcain@quicinc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Philippe Mathieu-Daudé <philmd@linaro.org>