powerpc: Don't write protect kernel text with CONFIG_DYNAMIC_FTRACE enabled

This problem was noticed on an MPC855T platform. Ftrace did oops
when trying to write to the kernel text segment.

Many thanks to Joakim for finding the root cause of this problem.
Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Joakim Tjernlund <joakim.tjernlund@transmode.se>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/include/asm/pte-common.h

@@ -162,7 +162,7 @@ extern unsigned long bad_call_to_PMD_PAGE_SIZE(void);
  * on platforms where such control is possible.
  */
 #if defined(CONFIG_KGDB) || defined(CONFIG_XMON) || defined(CONFIG_BDI_SWITCH) ||\
-	defined(CONFIG_KPROBES)
+	defined(CONFIG_KPROBES) || defined(CONFIG_DYNAMIC_FTRACE)
 #define PAGE_KERNEL_TEXT	PAGE_KERNEL_X
 #else
 #define PAGE_KERNEL_TEXT	PAGE_KERNEL_ROX