[POWERPC] Do firmware feature fixups after features are initialised

On pSeries the firmware features are not setup until ppc_md.init_early,
so we can't do the firmware feature sections fixups till after this.

Currently firmware feature sections is only used on iSeries which inits
the firmware features much earlier.  This is a bug in waiting on
pSeries.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>