fbdev: rename imacfb.txt to efifb.txt and change imacfb to efifb.

Rename imacfb.txt to efifb.txt since imacfb was moved to efifb,and change
imacfb to efifb.

Signed-off-by: Justin P. Mattock <justinmattock@gmail.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>