mpc5200: a3m071/a4m2k: Fix problem with increased global_data struct

The v2013.04 release has this patch set included:

5cb48582 "Add architecture-specific global data"

With this, the global_data struct is now common and new variables
have been added. Resulting in a bigger struct. Unfortunately the
currently allocated 128 bytes are just a bit too small for this
new struct.

This patch now uses the automatically generated struct size instead to
not run into this problem again.

Please note that this problem might hit some other platforms which
currently reserve a tight space of 128 bytes for the global_data
struct!

Signed-off-by: Stefan Roese <sr@denx.de>