riscv: Prevent a bad reference count on CPU nodes

In the Linux kernel, the following vulnerability has been resolved:

riscv: Prevent a bad reference count on CPU nodes

When populating cache leaves we previously fetched the CPU device node

at the very beginning. But when ACPI is enabled we go through a

specific branch which returns early and does not call 'of_node_put' for

the node that was acquired.

Since we are not using a CPU device node for the ACPI code anyways, we

can simply move the initialization of it just passed the ACPI block, and

we are guaranteed to have an 'of_node_put' call for the acquired node.

This prevents a bad reference count of the CPU device node.

Moreover, the previous function did not check for errors when acquiring

the device node, so a return -ENOENT has been added for that case.