ppc: Remove reloc_off field from global_data structure

Now that proper relocation is supported, the reloc_off field is no longer
necessary.

Note that the location of the standalone application jump table pointer
in the global data structure is affected by this change, breaking
execution of standalone applications compiled for previous versions of
U-Boot.

We therefore increment XF_VERSION to 6

Signed-off-by: Peter Tyser <ptyser@xes-inc.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>