MAINTAINERS: add IRC to ARM sub-architectures and Devicetree

Mention the IRC channels used for discussions about ARM/ARM64
sub-architectures and Devicetree.  This documents purely existing state.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220207175503.425200-2-krzysztof.kozlowski@canonical.com

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>