MAINTAINERS: Add Toolchain Integration entry

This commit adds the MAINTAINERS entry for the Toolchain Integration,
with @tejlmand as the maintainer and @stephanosio as a collaborator.

The purpose of this is to provide a corresponding MAINTAINERS entry for
the "area: Toolchains" and differentiate the toolchain integration-
related maintenance tasks from the general build system maintenance
tasks.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>