- When CONFIG_DISPLAY_CPUINFO is not enabled,
   print_cpuinfo() should be defined as an empty function
   in a header, include/common.h

 - Remove #ifdef CONFIG_DISPLAY_CPUINFO .. #endif
   from caller, common/board_f.c and arch/arm/lib/board.c

 - Remove redundant prototypes in arch/arm/lib/board.c,
   arch/arm/include/asm/arch-am33x/sys_proto.h and
   board/nokia/rx51/rx51.h, keeping the one in include/common.h

 - Add #ifdef CONFIG_DISPLAY_CPUINFO to the func definition
   where it is missing

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>