[POWERPC] Add definition of Cell bookmark SPR

Add a definition for the Cell bookmark SPR to asm-powerpc/regs.h.

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>