MAINTAINERS: Update lib/optee with my details

Commit 32ce6179 ("optee: Add lib entries for sharing OPTEE code across
ports") adds code into lib/optee but neglects to update MAINTAINERS to make
me buggable for questions and maintenance.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Suggested-by: Jens Wiklander <jens.wiklander@linaro.org>