[POWERPC] Remove old includes from arch/ppc

Remove includes of files that existed in arch/ppc that we dont need in
arch/powerpc anymore.  The following includes were removed:

 <asm/amigappc.h>
 <asm/bootinfo.h>
 <asm/ppcboot.h>
 <asm/ppc_sys.h>
 <asm/residual.h>
 <asm/m8260_pci.h>

This also caused platforms/embedded6xx/mpc7448_hpc2.h to no longer be
needed and thus removed.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>