ppc4xx: Fix TLB reset problem with recent 44x images

ppc4xx: Fix TLB reset problem with recent 44x images

Patch d873133f [ppc4xx: Add Sequoia RAM-booting target] broke "normal"
booting on some 44x platforms. This breakage is only noticed in some
cases while powercycling. As it seems, the code in question in start.S
didn't invalidate TLB #0. This makes sense since this TLB is used for
the bootrom mapping. With the patch mentioned above even TLB #0 got
invalidated resulting in an error later on.

This patch now fixes this issue by only invalidating TLB #0 in the RAM-
booting case.

Tested succesfully on Sequoia and Canyonlands.

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