MAINTAINERS: Add linux-samsung-soc mailing list for Samsung

There's a Samsung-specific mailing list but it's not advertised in
MAINTAINERS - add it to the overall architecture to make it more
discoverable.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
[kgene.kim@samsung.com: added notice of 'moderated']
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>