cpu: bmips: fix probe to get the address

In the device tree, the address for cpu is located in
the node "cpus", not in the cpu node (for exemple cpu@0).
So when probing cpu, the cpu address must be read in the
cpu parent.

The commit "cpu: bmips: convert to use live dt"
(sha1: c444afbb)
change this behaviour and read the address in the
cpu node when probing cpu.

We fix this by reading the address in the cpu parent.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>