powerpc: Pull common bits of setup_{32,64}.c into setup-common.c

This creates a new arch/powerpc/kernel/setup-common.c with various
bits that setup_32.c and setup_64.c had in common - functions like
machine_shutdown/restart/power_off, show_cpuinfo, set_preferred_console
etc.

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