firmware: PSCI: Fix PSCI support for OF live trees

When CONFIG_OF_LIVE is enabled, dev_of_offset() cannot be used and
if used returns an invalid offset. This causes the call to
fdt_stringlist_get() in the psci_probe() to fail to read the 'method'
property from the PSCI node for the device and hence prevents PSCI
from working. Fix this by using the ofnode_read_string() API instead
of the fdt_stringlist_get() because this will handle reading the
property both when CONFIG_OF_LIVE is enabled or disabled.

Due to the above problem and since commit 81ea0083 ("efi_loader:
PSCI reset and shutdown") was added, the EFI system reset has been
broken for Tegra210 and Tegra196 platforms. This also fixes the EFI
system reset for these Tegra platforms.

Signed-off-by: Jon Hunter <jonathanh@nvidia.com>
Tested-by: Peter Robinson <pbrobinson@gmail.com>