Allow setting NOT_COHERENT_CACHE explicitly.

Signed-off-by: Risto Suominen <Risto.Suominen@gmail.com>
---
The testing is done on kernel version 2.6.24.

--- a/arch/powerpc/platforms/powermac/Kconfig.org	2008-01-25 00:58:37.000000000 +0200
+++ b/arch/powerpc/platforms/powermac/Kconfig	2009-02-10 17:44:24.000000000 +0200
@@ -18,4 +18,10 @@ config PPC_PMAC64
 	select PPC_970_NAP
 	default y
 
-
+config NOT_COHERENT_CACHE
+	bool "Incoherent cache"
+	default n
+	help
+	  Setting this option may be necessary for avoiding cache-related
+	  problems with some network cards on some platforms. An example is
+	  2104x and PowerMac 5500.