ARM: SPEAr: Update MAINTAINERS entries

This merges few entries for the SPEAr platform as separate entries
aren't really required.

Moreover it adds soc@kernel.org as 'M' entry, to let them be cc'd for
patches, since they only pickup the patches for SPEAr directly due to
very low traffic for the same.

Suggested-by: Rob Herring <robh@kernel.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>