MAINTAINERS: Add myself as TEE subsystem reviewer

Since I have been helping with TEE subsystem reviews, so make that role
official.
Signed-off-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Jens Wiklander <jens.wiklander@linaro.org>

MAINTAINERS

@@ -17916,6 +17916,7 @@ F:	include/media/i2c/tw9910.h
 
 TEE SUBSYSTEM
 M:	Jens Wiklander <jens.wiklander@linaro.org>
+R:	Sumit Garg <sumit.garg@linaro.org>
 L:	op-tee@lists.trustedfirmware.org
 S:	Maintained
 F:	Documentation/staging/tee.rst