MAINTAINERS: add a limited ARM and ARM64 SoC entry

It is expected for ARM and ARM64 SoC related code to go through
sub-architecture maintainers.  Their addresses were therefore not
documented to push patch traffic through sub-architecture maintainers.

However when patches touch generic code, e.g. multi_v7_defconfig, the
patch might not be picked up by them and instead should go to the SoC
maintainers - Arnd and Olof.

Add a minimal maintainer's entry for SoC covering only Makefile, so it
will not appear on most of submissions (except new devicetree boards).
It will though serve as a documentation and reference for cases when
submitter does not know where to send his SoC-related patches.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20201201211516.24921-2-krzk@kernel.org

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