[POWERPC] Add kernel parameter to set l3cr for MPC745x

Old-world powermacs don't set L2CR or L3CR on processor upgrade cards.
This simple patch allows the setting of L3CR via a kernel parameter
(like the existing kernel parameter to set L2CR).

Signed-off-by: Robert Brose <bob@qbjnet.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>