ARM: debug-ll: rework footbridge handling

Footbridge has two debug ports that are handled a bit differently:

The 8250 port uses the normal debug/8250.S implementation that is shared
with a lot of other platforms, but it relies on the DEBUG_UART_8250
option to be turned on automatically instead of being selected by
DEBUG_FOOTBRIDGE_COM1 as we do for most other platforms. I'm changing
this to use a 'select' and change the dependency to the debug symbol
rather than the platform symbol for consistency.

The DC21285 UART has a separate top-level option, and relies on
the traditional include/mach/debug-macro.S method. With the s3c64xx
multiplatform series queued up for 4.5, it is now the last one that does
this, so by moving this file to include/debug/dc21285.S, we can get
all platforms to do things the same way.

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