powerpc: Add new CPU feature: CPU_FTR_CP_USE_DCBTZ

Add a new CPU feature bit, CPU_FTR_CP_USE_DCBTZ, to be added to the
64bit powerpc chips that benefit from having dcbt and dcbz
instructions used in their memory copy routines.

This will be used in a subsequent patch that updates copy_4K_page().
The new bit is added to Cell, PPC970 and Power4 because they show
better performance with the new copy_4K_page() when dcbt and dcbz
instructions are used.

Signed-off-by: Mark Nelson <markn@au1.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>