MAINTAINERS: Add dts/vendor/ti to TI K3 Platforms

This directory was added for the DT items common to both ARM (dts/arm) and
ARM64 (dts/arm64) cores for TI devices.

For now the only TI devices that share DT nodes between cores of different
architectures is the K3 family. If other TI devices start using this dir
then we can get more granular in the MAINTAINERS entry here. Until then
mark the whole directory as part of "TI K3 Platforms".

Signed-off-by: Andrew Davis <afd@ti.com>