soc/tegra: pmc: Fix early initialisation of PMC

During early initialisation, the available power partitions for a given
device is configured as well as the polarity of the PMC interrupt. Both
of which should only be configured if there is a valid device node for
the PMC device. This is because the soc data used for configuring the
power partitions is only available if a device node for the PMC is found
and the code to configure the interrupt polarity uses the device node
pointer directly.

Some early device-tree images may not have this device node and so fix
this by ensuring the device node pointer is valid when configuring these
items.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>