powerpc/47x: Disable 256k page size

PPC47x_TLBE_SIZE isn't defined for 256k pages, leading to a build
break if 256k pages is selected.

So change the kconfig so that 256k pages can't be selected for 47x.

Fixes: e7f75ad0 ("powerpc/47x: Base ppc476 support")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Expand change log to mention build break]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/2fed79b1154c872194f98bac4422c23918325e61.1611128938.git.christophe.leroy@csgroup.eu

arch/powerpc/Kconfig

@@ -768,7 +768,7 @@ config PPC_64K_PAGES
 
 config PPC_256K_PAGES
 	bool "256k page size"
-	depends on 44x && !STDBINUTILS
+	depends on 44x && !STDBINUTILS && !PPC_47x
 	help
 	  Make the page size 256k.