MAINTAINERS: use Krzysztof Kozlowski's Canonical address

Since I plan to use my Canonical address for reviews and other
maintenance activities, reflect this in MAINTAINERS to avoid any
confusion.

Cc: Krzysztof Kozlowski <krzk@kernel.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Olof Johansson <olof@lixom.net>
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20210304075751.9201-1-krzysztof.kozlowski@canonical.com