powerpc/mpc512x: remove unnecessary #if

Several functions are only ever referenced locally, so make them static.
Of those functions, many of them are protected by an #if. However, the
code which can compile fine in either case.

Now that (1) the unneeded code is marked 'static' and (2) the code is
only used under a C 'if (IS_ENABLED(CONFIG_FB_FSL_DIU))', the compiler
can automatically remove the unneeded code, and we don't need the #if or
the empty stub functions.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>