broadsheetfb: add MMIO hooks

Allow boards with GP-MMIO controllers to provide hooks to broadsheetfb in
order to offload cmd/data writes and data reads instead of relying only on
host based GPIO wiggling.

Signed-off-by: Jaya Kumar <jayakumar.lkml@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>