[POWERPC] Fix kernel panic in arch_arm_kprobe

The code in arch_arm_kprobe was trying to set a breakpoint which
resulted in a page fault because the kernel text pages were write
protected.  Disable the write protect when CONFIG_KPROBES is defined.

Signed-off-by: Ionut Nicu <ionut.nicu@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>