powerpc: Merge remaining RTAS code

This moves rtas-proc.c and rtas_flash.c into arch/powerpc/kernel, since
cell wants them as well as pseries (and chrp can use rtas-proc.c too,
at least in principle).  rtas_fw.c is gone, with its bits moved into
rtas_flash.c and rtas.c.

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