powerpc/512x: don't compile any platform DIU code if the DIU is not enabled

If the DIU framebuffer driver is not enabled, then there's no point in
compiling any platform DIU code, because it will never be used.  Most of
the platform code was protected in the appropriate #ifdef, but not all.
This caused a break in some randconfig builds.

This is only a problem on the 512x platforms.  The P1022DS and MPC8610HPCD
platforms are already correct.

This patch reverts commit 12e36309 ("powerpc:
Option FB_FSL_DIU is not really optional for mpc512x") and restores the
ability to configure DIU support.

Signed-off-by: Timur Tabi <timur@freescale.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>