powerpc/mpc512x: silence build warning upon disabled DIU

powerpc/mpc512x: silence build warning upon disabled DIU

a disabled Kconfig option results in a reference to a not implemented
routine when the IS_ENABLED() macro is used for both conditional
implementation of the routine as well as a C language source code test
at the call site -- the "if (0) func();" construct only gets eliminated
later by the optimizer, while the compiler already has emitted its
warning about "func()" being undeclared

provide an empty implementation for the mpc512x_setup_diu() and
mpc512x_init_diu() routines in case of the disabled option, to avoid the
compiler warning which is considered fatal and breaks compilation

the bug appeared with commit 2abbbb63c90ab55ca3f054772c2e5ba7df810c48
"powerpc/mpc512x: move common code to shared.c file", how to reproduce:

  make mpc512x_defconfig
  echo CONFIG_FB_FSL_DIU=n >> .config && make olddefconfig
  make

    CC      arch/powerpc/platforms/512x/mpc512x_shared.o
  .../arch/powerpc/platforms/512x/mpc512x_shared.c: In function 'mpc512x_init_early':
  .../arch/powerpc/platforms/512x/mpc512x_shared.c:456:3: error: implicit declaration of function 'mpc512x_init_diu' [-Werror=implicit-function-declaration]
  .../arch/powerpc/platforms/512x/mpc512x_shared.c: In function 'mpc512x_setup_arch':
  .../arch/powerpc/platforms/512x/mpc512x_shared.c:469:3: error: implicit declaration of function 'mpc512x_setup_diu' [-Werror=implicit-function-declaration]
  cc1: all warnings being treated as errors
  make[4]: *** [arch/powerpc/platforms/512x/mpc512x_shared.o] Error 1

Signed-off-by: Gerhard Sittig <gsi@denx.de>
CC: <stable@vger.kernel.org> # v3.11
Signed-off-by: Anatolij Gustschin <agust@denx.de>