MAINTAINERS: Add Toolchain Integration entry
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 <dc76e9f0c0006e8f919e0c515c66dbba3982f785@stephanos.io>

zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr