[POWERPC] Always apply DABR changes on context switches

This patch removes the #ifdef CONFIG_PPC64 around setting the DABR.

The actual setting of the SPR inside of the set_dabr() function is dependent
on CONFIG_PPC64 || CONFIG_6xx but you can always provide a ppc_md hook to
override that.  We should improve support for different HW breakpoints
facilities but this is a first step.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>