ARM: s3c: move into a common directory

s3c24xx and s3c64xx have a lot in common, but are split across three
separate directories, which makes the interaction of the header files
more complicated than necessary.

Move all three directories into a new mach-s3c, with a minimal
set of changes to each file.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[krzk: Rebase, add s3c24xx and s3c64xx suffix to several files, add SPDX
headers to new files, remove plat-samsung from MAINTAINERS]
Co-developed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
https://lore.kernel.org/r/20200806182059.2431-39-krzk@kernel.org