powerpc/powernv: Initialise nest mmu

powerpc/powernv: Initialise nest mmu

BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1675806
POWER9 contains an off core mmu called the nest mmu (NMMU). This is
used by other hardware units on the chip to translate virtual
addresses into real addresses. The unit attempting an address
translation provides the majority of the context required for the
translation request except for the base address of the partition table
(ie. the PTCR) which needs to be programmed into the NMMU.

This patch adds a call to OPAL to set the PTCR for the nest mmu in
opal_init().

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
(cherry picked from commit 1d0761d2557d1540727723e4f05395d53321d555)
Signed-off-by: Breno Leitao <breno.leitao@gmail.com>
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>