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

This is an implementation of commits 53a712ba
("powerpc/powernv/idle: Restore AMR/UAMOR/AMOR after idle") and
a3f3072d ("powerpc/powernv/idle: Restore IAMR after idle") using
the new C-based idle code.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Extract from Nick's patch]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>