powerpc: Merge in the ppc64 version of the prom code.

This brings in the ppc64 version of prom_init.c, prom.c and btext.c
and makes them work for ppc32.  This also brings in the new calling
convention, where the first entry to the kernel (with r5 != 0) goes
to the prom_init code, which then restarts from the beginning (with
r5 == 0) after it has done its stuff.

For now this also brings in the ppc32 version of setup.c.  It also
merges lmb.h.

Signed-off-by: Paul Mackerras <paulus@samba.org>