[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>

Documentation/kernel-parameters.txt

@@ -931,6 +931,8 @@ and is between 256 and 4096 characters. It is defined in the file
 
 	l2cr=		[PPC]
 
+	l3cr=		[PPC]
+
 	lapic		[X86-32,APIC] Enable the local APIC even if BIOS
 			disabled it.