powerpc: Add the DAWR support to the set_break()

This adds DAWR supoprt to the set_break().

It does both bare metal and PAPR versions of setting the DAWR.

There is still some work we can do to make full use of the watchpoint but that
will come later.

Signed-off-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>