soc/tegra: pmc: Update scratch as an optional aperture

Scratch address space register is used to store reboot reason. For
some Tegra234 systems, the scratch space is not available to store
the reboot reason. This is because scratch region on these systems
is not accessible by the kernel as restricted by the Hypervisor.
Such systems would delist scratch aperture from PMC DT node.

Hence this change makes scratch as optional aperture and also avoids
registering reboot notifier if scratch address space isn't mapped.

Signed-off-by: Petlozu Pravareshwar <petlozup@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>