powerpc/powernv/idle: Restore AMR/UAMOR/AMOR after idle

mainline inclusion
from mainline-v5.2-rc1
commit 53a712ba
category: bugfix
bugzilla: 13690
CVE: CVE-2020-11669

-------------------------------------------------

In order to implement KUAP (Kernel Userspace Access Protection) on
Power9 we will be using the AMR, and therefore indirectly the
UAMOR/AMOR.

So save/restore these regs in the idle code.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>