MAINTAINERS: Add reviewer for ACTIONS platforms

Since I'll be working on improving support for ACTIONS platforms, adding
myself as the reviewer.

Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Andreas Färber <afaerber@suse.de>