PM / devfreq: memlat: Get complete CPU list during the probe

Currently arm-memlat-mon driver uses cpu_coregroup_mask to get
the sibling CPUs for the first CPU specified during the probe.
However if any of the CPUs in a cluster isn't hotplugged in
even once before the probe runs then the call to cpu_coregroup_mask
might not give the updated list of all the sibling CPUs for the
specified CPU.
With this change the initialization of the cpumask for the memlat
device is obtained from the list of CPU phandles specified in the
dtsi file.

Change-Id: Ide97d60d9eecbbe1d33deda72a13951059822896
Signed-off-by: Rohit Gupta <rohgup@codeaurora.org>