ARM: s3c: make headers local if possible

A lot of header files are only used internally now, so they can be moved
to mach-s3c, out of the visibility of drivers.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200806182059.2431-40-krzk@kernel.org
[krzk: Rebase and fixup leds-s3c24xx driver]
Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>