soc/tegra: pmc: Restrict legacy code to 32-bit ARM

For backwards-compatibility with old device trees, if no PMC node exists
this driver hard-codes the I/O memory region. All 64-bit ARM device tree
files are recent enough that they can be required to have this node, and
therefore the legacy code path is not required on 64-bit ARM.

Based on work done by Paul Walmsley <pwalmsley@nvidia.com>.

Cc: Paul Walmsley <pwalmsley@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>