[POWERPC] spufs: Add bit definition

Add a bit define from book, and replace one hex number with a
symbol, for clarity.

Signed-off-by: Sebastian Siewior <bigeasy@linux.vnet.ibm.com>
Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>