MAINTAINERS: correct SoC Git address (formerly: arm-soc)

The SoC Git was moved from arm/arm-soc.git to soc/soc.git.  Correct the
ARM Sub-architectures entry.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201201211516.24921-1-krzk@kernel.org

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