[POWERPC] PS3: Move chip mask defs up

This just moves the definitions of the PS3 chip_mask routines up
above the irq setup routines.  This change is needed for the
kexec updates that follow.  Also adds some inline documentation
to the routines.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>