msm: spm-devices: Use phandle to identify the cpu vctl device.

Voltage change requests for a cpu rail need to be handled by a
spm device that is capable of it. Use a list of cpu phandle to
identify which spm device can handle a cpu's voltage request.
Use the voltage control capable spm device to route the voltage
change request.

Older targets may not support the use of phandles. Such targets
can provide a cpumask as an entry in the spm device node in
place of a phandle.

With the ability to dynamically lookup the voltage control spm
device, there is no need to specifically save the l2 spm device
data. Remove all references to l2_spm_device.

Change-Id: I44182c6bce8c53b40c865036e45177ad15cb52e1
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>
Signed-off-by: Murali Nalajala <mnalajal@codeaurora.org>