MAINTAINERS: arm: samsung: add Git tree and IRC

Add already used Krzysztof Kozlowski's Git tree for Samsung
S3C/S5P/Exynos ARM sub-architecture and IRC channel (#linux-exynos at
Libera).  This documents purely existing state.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Link: https://lore.kernel.org/r/20220207175503.425200-1-krzysztof.kozlowski@canonical.com

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