ppc4xx: Fix platform support

ppc4xx: Fix platform support

Commit "ecc3066 Fix board init code to respect the C runtime environment"
broke platform support for ppc4xx.
start.S prepares a stackframe that is later rendered unusable by appending
the reserved space for global data.
Instead the reserved space has to be put first. Then the stackframe can
be pushed.

I can only test the 405EP OCM case. At least all other ppc4xx boards still
build.

Signed-off-by: Dirk Eibach <dirk.eibach@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>