powerpc: mpc83xx: Allow including initreg.h into multiple files

powerpc: mpc83xx: Allow including initreg.h into multiple files

Globals defined in headers can result in multiple-definition errors
while linking, if they are visible beyond the current translation unit.

This hasn't been a problem for initreg.h so far, but would become a
problem in the next patch, where I use a constant from initreg.h in a
second C file.

Reviewed-by: Sinan Akman <sinan@writeme.com>
Signed-off-by: J. Neuschäfer <j.ne@posteo.net>