Staging: fbtft: add header for internal functions

Remove extern keyword from function prototypes to suppress warning from
checkpatch.pl with --strict option: https://lkml.org/lkml/2013/7/23/422



fbtft maintainer Noralf Tronnes advised these functions are internal to
this module & suggested moving these prototypes to new internal.h file.
He also advised fbtft.h file will eventually live in include/linux/fbtft.h

Suggested-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Drew Fustini <pdp7pdp7@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>