ppc: Use r12 instead of r14 as GOT pointer.

r14 is not supposed to be clobbered by functions. Switch
to r12 and call GET_GOT when needed. This will allow u-boot
to loose the -ffixed-r14 gcc option.

Signed-off-by: Joakim Tjernlund <Joakim.Tjernlund@transmode.se>