ARM: tegra: use setup_mm_for_reboot rather than explicit pgd switch

This patch changes the Tegra PM code to use the setup_mm_for_reboot
helper rather than call cpu_switch_mm directly. This keeps things like
TLB invalidation in one place.

Cc: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>