microblaze: remove duplicated #include's

Remove duplicated #include's in
  arch/microblaze/include/asm/io.h
  arch/microblaze/kernel/prom.c
  arch/microblaze/kernel/ptrace.c
  arch/microblaze/kernel/signal.c
  arch/microblaze/kernel/sys_microblaze.c

Signed-off-by: Huang Weiyi <weiyi.huang@gmail.com>
Signed-off-by: Michal Simek <monstr@monstr.eu>