printk: move braille console support into separate braille.[ch] files

Create files with prototypes and static inlines for braille support.  Make
braille_console functions return 1 on success.

Corrected CONFIG_A11Y_BRAILLE_CONSOLE=n _braille_console_setup
return value to NULL.

Signed-off-by: Joe Perches <joe@perches.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Cc: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>