soc: officially expand maintainership team

Since Olof moved on from the soc tree maintenance, Arnd has mainly taken
care of the day-to-day activities around the SoC tree by himself, which
is generally not a good setup.

Krzysztof, Linus and Alexandre have volunteered to become co-maintainers
of the SoC tree, with the plan of taking turns to do merges and reviews
to spread the workload. In addition, Drew joins as another reviewer.

Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Drew Fustini <fustini@kernel.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>