[POWERPC] Fix Kexec/Kdump for power6

On Power machines supporting VRMA, Kexec/Kdump does not work.
VRMA (virtual real-mode area) means that accesses with IR/DR = 0
(i.e. the MMU "off") actually still go through the hash table,
using entries put there by the hypervisor.

This means that when we clear out the hash table on kexec, we need to
make sure these entries are left untouched.

This also adds plpar_pte_read_raw() on the lines of
plpar_pte_remove_raw().

Signed-off-by : Sachin Sant <sachinp@in.ibm.com>
Signed-off-by : Mohan Kumar M <mohan@in.ibm.com>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Olof Johansson <olof@lixom.net>

Signed-off-by: Paul Mackerras <paulus@samba.org>