malloc: remove extern declarations of malloc_bin_reloc() in board.c files

Declare malloc_bin_reloc() in malloc.h and remove all extern declarations
in various board.c files to get rid of one checkpatch.pl warning.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Andreas Bießmann <andreas.devel@gmail.com>
Cc: Jason Jin <Jason.jin@freescale.com>
Cc: Macpaul Lin <macpaul@andestech.com>
Cc: Daniel Hellstrom <daniel@gaisler.com>
Acked-by: Andreas Bießmann <andreas.devel@gmail.com>