ARM: mvebu: remove device tree parsing for cpu nodes

Currently set_secondary_cpus_clock assume the CPU logical ordering
and the MPDIR in DT are same, which is incorrect.

Since the CPU device nodes can be retrieved in the logical ordering
using the DT helper, we can remove the devices tree parsing.

This patch removes DT parsing by making use of of_get_cpu_node.

Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Jason Cooper <jason@lakedaemon.net>
Acked-by: Gregory Clement <gregory.clement@free-electrons.com>
Signed-off-by: Sudeep KarkadaNagesha <sudeep.karkadanagesha@arm.com>